The shrubs and trees of the Tamaulipan thornscrub in the semiarid regions of the northeastern Mexico are of great economic importance for various uses such as timber for furniture, fence, post, firewood and sources of forage for wild grazing animals for possessing macro and micro-nutrients required by animals, herbs, medicine and reforestation [1]. The utilization of wood for different purposes depends on the structure, physical and chemical characteristics of wood elements, mainly fiber cells. Studies have been undertaken on these aspects by researchers. Wood is a hard, fibrous structural tissue present in the stems and roots of woody species. Its main use is for furniture and building construction, besides fences, paper pulp and other products. With respect to its structure, wood is the secondary xylem derived by intercalary meristem consisting of cambium in the stems of trees. Cambium consists of two types of cells: fusiform cells giving fiber cells and xylem vessels, and the ray initials gives ray cells forming medullary cells. Fiber cells are composed of cellulose fibrils embedded in a matrix made of lignin, which resists compression. It gives mechanical strength to the plant and also helps to transfer water and nutrients to the leaves and other growing tissues.

Wood fibers have a complex ultrastructure and are composed of several cell wall layers. These cell wall layers vary in the contents of cellulose, hemicelluloses and lignin. The chemistry and ultrastructure of wood components determines the properties of this lignocellulosic fiber of perennial plants, and also determine longitudinal growth stress generated during cell-wall maturation as a result of the biosynthetic and biochemical processes during cell wall formation. There exist various types of reaction wood revealing extreme cases of macromolecular and ultra-structural organization. Approximately half of the angiosperms species produce tension wood where the secondary wall is partially replaced by a so-called “gelatinous layer” where lignin is absent and made up of axially oriented cellulosic microfibrils. The final fiber properties contribute greatly to the quality of pulp and paper as well as timber and its products.

The major components of plant cell walls are cellulose, hemicelluloses and lignin. A cellulose chain consists of about 10,000 glucose units. In general, 30-40 parallel cellulose chains associate to form microfibrils containing crystalline and amorphous parts. The orientation of microfibrils in the various cell wall layers contributes to the strength properties of individual fibers and wood [2-4].

Fujiwara and Yangk [5] reports there exists relationship between cell length and ring width and circunferencial growth rate in Canadian species.

Carillo-Parra et al. [6] studied differences of wood elements (fiber cell and vessels) of Prosopis sp. in two locations of northeast Mexico and reported variation in fiber cell and xylem vessel length depending on precipitation and temperature. The location with high precipitation produced longer fiber length compared to other location having low precipitation and high temperature. The results reveal a gradual increase of the first two parameters with increase in distance from the pith to the periphery of the trunk and an inverse relation between for fiber diameter and the lumen diameter [7].

Barnett and Bonham [8] studied the cellulose microfibril angle in the cell wall of wood fibers. The term microfibril angle (MFA) in wood science is defined as the angle between the direction of the helical windings of cellulose microfibrils in the secondary cell wall of fibers and tracheids and the long axis of cell. The cellulose microfibrils are oriented in the S2 layer of the cell walls that forms the greatest proportion of the wall thickness and is responsible for the physical properties of wood. The authors made a review on the organization of the cellulose component of the secondary wall of fibers and tracheids and the various methods that have been used for the measurement of MFA. They observed a large variation of MFA within the tree observed between juvenile (or core) wood and mature (or outer) wood. These differences in MFA have an effect on the properties of wood, with respect to its stiffness. The large MFA in juvenile wood contributes to low stiffness and gives the sapling the flexibility necessary to survive high winds without breaking. This is of great importance in forestry for short rotation cropping of fast growing species. These species are presently grown mainly for pulp. The mechanism for the orientation of microfibril deposition is not yet fully known [8].

Coops et al. [9] made a critical assessment of standing wood and fiber quality using ground and airborne laser scanning. Physical and chemical characteristics of wood shows variations with both tree and site characteristics and also on the crown development, stem shape and upper layer, branch size and branch location, knot size, type and placement, age. All these characteristics influence wood properties but also on stocking density, moisture, nutrient availability, climate, competition, disturbance, and stand age have also been confirmed as key determinants of wood quality. They identified a number of key wood quality attributes (i.e. basic wood density, cell perimeter, cell coarseness, fiber length, and microfibril angle) and established links between these properties and forest structure and site attribute. This technique is recommended to predict wood quality in standing timber [9].

The properties of wood and wood based materials are strongly dependent on the properties of its fibers; i.e., the cell wall properties. Therefore, the ability to characterize these in order to increase our understanding of structure-property relationships is very important to examine. The authors undertook a brief overview of the state of the art in experimental techniques to characterize the mechanical properties of wood at both the level of the single cell and that of the cell-wall [10].

Sample woods for fiber characterization. One disk of 0.1 m thick was taken from two primary branch of a tree from each species. Few small pieces of wood of each species were dipped in concentrated nitric acid and plugged with cotton. Then the test tubes are kept in boiling water bath until the wood pieces started disintegrating. Then acid was decanted slowly and the macerated wood elements were washed several times with distilled water. Then, the macerated fiber cells were stained with safranin (1%) and observed under microscope and taken photographs with digital camera fixed with microscope. Fifty fiber cells of each species were measured using ocular and stage micrometer.

In the following are depicted the morphology of fiber cells of woods in the woody species of Tamaulipan thornscrub.

Wood fibers

Acacia berlandieri: The apex is pointed to round, the lumen somewhat broad, the cell wall is thin, but little lignified. Wood is semihard for fabrication of furniture semi-hard and paper.

Acacia farnesiana: Some fiber cells are non-uniform, few are uniform, apex pointed, the lumen is broad, cell wall is medium thick but little lignified.

Bernardia myricifolia: The fiber cells are thin, moderately long, apex pointed, cell wall is medium thick.

Caesalpinia Mexicana: The fiber cells are uniform, wide, medium long, apex pointed, lumen broad, uniform. Cell wall is thin. Good for paper pulp.

Capsicum annuum: The fiber cells are long, apex pointed, cell wall is thin but little lignified. The lumen is little broad. Good for paper pulp.

Eysenhardtia polystachya: The fiber cells are uniform, apex is pointed, cell wall is thin, the lumen is little broad, suitable for paper.

Forestiera angustifolia: The fiber cells are long, the apex is pointed, cell wall is medium thin, and lumen is broad, suitable for soft furniture and paper.

Fouquiera splendens: The fiber cells are small or long, thin, apex pointed or round, cell wall thick, lumen is narrow, suitable for strong furniture.

Helietta parvifolia: The apex is pointed, cell wall is medium thick, and the lumen is thin. Good for soft furniture.

Lantana macropoda: The fiber cells are small, long, thin walled but little lignified, apex pointed or round, the lumen is little broad, suitable for soft furniture and paper.

Leucophyllum frutescens: The fiber cells are small or long with pointed or round apex, the lumen is narrow, the cell wall is thin to medium. Good for soft furniture.

Morus celtidifolia: The fiber cells are thin, uniform or non-uniform, apex is pointed, the lumen is little broad. The cell wall is medium. Good for paper pulp.

Quercus virginiana: The fiber cells are somewhat long, thin, majority is uniform, the apex is pointed or round. The cell wall is thin. Good for paper pulp.

Salvia officinalis: The fiber cells are small, or long and thin, cell wall thin, but little lignified, the lumen is narrow. Suitable for soft furniture.

Tribulus terrestres: The apex is pointed, cell wall is medium thick walled hin, and the lumen is narrow.

The species show large variation in fiber cell length, breadth and wall thickness as shown in the Table 1.

Table 1: Average and standard deviation of fibre cell length, diameter and wall thickness of Tamulipan thorn scrub in Linares, Northeast of Mexico.

Figure 1: Wood fiber cells at 10 × and 40 ×.

The main objective of this present study is to investigate the variability in the morphology, size, shape and dimensions of fibers of 14 woody species and one shrub. No attempts have been made to its physical quality and ultrastructure of the fiber cells. Many studies have been directed on the growth, ultrastructure and orientation of microfibrils in fiber cell [4].

In the present study large variations were observed in the size, shape of fiber cells among woody and one shrub. On the basis of this we classify the species on the morphological characteristics of the fiber cells of the species studied.

Cell wall thin: Acacia berlandieri, Bernardia myricifolia, Helietta parvifolia, Leucophyllum frutescens, Quercus virginiana, and Tribulus terrestres. The woods of these species may recommend for soft furniture, fences and paper pulp.

Cell wall thin, lumen broad: Acacia berlandieri, Caesalpinia mexicana, Eysenhardtia polystachya, Forestiera angustifolia, Lantana macropoda, and Morus celtidifolia. The wood of these species may be recommended for the preparation of paper pulp and good quality paper.

Cell wall thick, lignified, lumen narrow: Fouquiera splendens and Salvia officinalis. These species may be recommended for furniture and construction.

In the present study there were large variations in fiber cell length among different wood species on the basis of we can tentatively classify different species.

Fiber cell mediumly long: Fouquiera splendens (563 μm), Forestiera angustifolia (488 μm), Eysenhardtia polystachya (478 μm), Acacia farnesiana (473 μm), Acacia berlandieri (464 μm). It is expected that the wood of these species may offer moderate strength of wood and its products.

Fiber cells long: Quercus virginiana (709 μm), Helietta parvifolia (648 μm), Tribulus terrestri (591 μm), Acacia farnesiana (473 μm), Morus celtidifolia (455 μm), Salvia officinalis (454 μm). It is expected that the timbers of these species may offer greater strength to the wood and its products and papers.

Fiber cells short (less than 45 μm). Lantana macropoda (392 μm), Capsicum annuum (437 μm), Leucophyllum frutescens (438 μm). It is expected these timbers may offer poor strength to its woods and its products.

The present study was limited only in one locality in order to study the variability in fiber cell morphology and fiber cell length which can vary in different locality, environments, and positions in the trees which have been documented by various authors.

We did not attempt to study the relationship between fiber cell length and timber quality which have been reported by various authors.

Fibre cell length is an important trait contributing the quality of a Wood [8,11,12]. Variation in fiber cell length, diameter and cell wall thickness are related to fiber quality and variations in different positions and its growth rate [11,13].

Artemillo et al. 2013 [14] reported that the variation in fiber and vessel length in Prosopis varied in two localities depending on the precipitation and temperature. High precipitation and low temperature contributed to higher fiber cell length. On the other hand radial variation in fiber cell length was reported in oak [12,15].

In conclusion it may be stated that we observes large variability in fiber cell morphology, fiber cell length and we classified species for their possible utility. Further studies are needed to confirm these hypotheses. There is also a necessity to study the variation in fiber cell length in different environment of the selected species. This is a potential line of research in forest science.

The authors are thankful to Elsa Gonzlez for hard labor in analyzing data in the laboratory.