764 ECOLOGY 



Compression strength. — The trunks of trees are subject to compression, owing 

 to the weight of the parts above. Longitudinal pressure of this sort requires 

 columnar strength. In many trees (as in the walnut, red cedar, and most conifers) 

 there is a solid supporting column whose central portions are the hardest and 

 strongest, but in some instances (as in the sycamore) the supporting column may 

 become a hollow cylinder, owing to the decay of the heartwood. 



Roots and rhizomes are subject to radial pressure as they grow in diameter. • A 

 peripheral mechanical cylinder is the most effective means of resisting such radial 

 pressure, and this is present in many rhizomes in addition to a central mechanical 

 strand. Prop roots (fig. 739) are subject to unusual strains, since stem flexion 

 causes severe tension on one side of the plant and equally severe compression on the 

 other. The prop roots of corn often contain both central strands and peripheral 

 cylinders of mechanical tissue, so that they are well fitted to withstand strains of 

 every sort. 



5. THE PROTECTIVE RELATIONS OF STEMS 



Introductory remarks. — The greatest of dangers to plants is excessive 

 transpiration, and to this the aerial stem is especially exposed; indeed, 

 in deciduous trees and shrubs the aerial stem is the most exposed of all 

 organs, since it alone is subject to transpiration and to other aerial 

 dangers during periods of drought or cold. Aerial stems are struc- 

 turally the best protected of plant organs, save only seeds. In many 

 cases the stems, as well as the leaves, are deciduous, and in such plants 

 protective structures usually are much less developed, their habit of life 

 constituting their chief protection. 



Epidermis. — In aerial herbaceous stems the protective tissues resemble those of 

 leaves (p. 567), the outer epidermal walls being highly cutinized, while waxy coats 

 and hairy coverings occur in many cases. In the European mistletoe {Viscum 

 album) and in one of the maples (Acer striatum), the epidermis, through continual 

 radial division, remains as a relatively permanent layer, a forty-year-old trunk of the 

 latter sometimes being covered with a true epidermis. 



Bark. — In most trees and shrubs the inception of secondary growth 

 is followed by the rupture and exfoliation of the epidermis and of 

 portions of the cortex. Thenceforth the protective tissues generally are 

 called bark, a term including the heterogeneous complex of living and 

 dead cells outside the cambium ring. Apart from remnants of the 

 epidermis and the primary cortex, the bark consists chiefly of the 

 phellogen and its products and of the secondary phloem, the latter 

 including new living cells near the cambium (sieve tubes, companion 

 cells, parenchyma), dead cells of similar character farther out, and 



