700 ECOLOGY 



mechanical tissue when the fruits hang free in the air than when tliey rest on the 

 ground. It is claimed by some investigators that compression is more effective than 

 tension in stimulating the growth of mechanical tissue. Roots react to mechanical 

 stimuli more readily than do stems, tension resulting in a. conspicuous increase in 

 the number of mechanical elements and in the thickness of the walls, and com- 

 pression resulting in a decreased cell size and in an increased wall thickness. 



If a young tree is fastened so that it can sway in but one plane, its diameter in 

 this plane will be greater than in any other. Probably the elliptical cross section 

 seen in trunks of seacoast trees is caused by wind, the long diameter being per- 

 pendicular to the coast line. In the spruce the upper side of a horizontal branch is 

 composed largely of white wood and the lower side of red wood, the former having 

 about twice the tensile strength of the latter; the red wood, whose elements have 

 thicker walls, possesses the greater compression strength. The upper part of a 

 branch is subjected to tension and the lower part to compression, and the differ- 

 ences observed may thus be explained; gravity also is believed to be an important 

 causative factor. Red wood occurs also on the lee side of branches exposed to 

 wind. In dicotylous trees the differences of wood are less marked than in conifers, 

 and mechanical tissues are, if anything, better developed on the upper than on the 

 under side of horizontal branches. 



Desiccation. — While the influence of mechanical stimuli is not clearly 

 understood, desiccation is known to favor the increased development of 

 mechanical tissue. In dry air a mechanical cylinder is developed in 

 the stem cortex of Mentha aguatica, while in moist air the cells remain 

 parenchymatous. In the stem of Ficus scandens the cells that become 

 coUenchyma in moist air become bast in dry air. In deserts plants of 

 irrigated soil show less mechanical tissue than do those of dry soil. 

 Aquatic and terrestrial stems of the same species (as Polygonum amphi- 

 hium, figs. 821, 822) differ widely in the amount of mechanical tissue 

 developed, the water form being too weak to stand alone, whereas the 

 air form is very stiff and rigid. Fern roots in moist soil have a slight 

 development of mechanical tissue, the cells being few and the walls thin. 

 Probably it is generally true that bast, collenchyma, and sclerenchyma 

 cells increase in number and in wall thickening as the tissues in which 

 they are developing are exposed increasingly to desiccation. The mini- 

 mum of mechanical tissue occurs where transpiration is reduced to zero, 

 namely, in the water. Observation confirms experiment, for mechanical 

 tissues are less developed in hydrophytic than in other habitats (figs. 791, 

 1018), reaching their culmination in xerophytes. 



The role of mechanical tissues. — The mechanical features of 

 stereids. — The fact that ropes and cables are made from the bast fibers 

 of hemp, ramie, etc., indicates the tensile strength of bast strands. 

 While the strength of ordinary parenchyma, as pith, is but one kiloj- 



