32 THE ECOLOGY OF STEMS 



Roots, on the other hand, do not need to be stiff. They need 

 rather to be able to resist a pull and, since the pull is not always 

 straight, they need to be able to bend without breaking. Accord- 

 ingly we find that the mechanical tissue of roots is often largely near 

 the center and that there is a thick cortex of thin-walled cells. 



As in the case of conducting tissues the chief factor influencing 

 the development of mechanical tissue is water, or rather, lack of 

 water. Many submerged water plants are almost entirely without 

 mechanical tissue. Such plants may entirely collapse as soon as 

 removed from the water. All seed plants that grow' out of the water, 

 however, have some mechanical tissue and this tissue reaches its 

 maximum development in plants growing in desert regions. 



20. Protective Tissues.— The greatest danger to which most land 

 plants are subjected is too great a loss of water. While roots are 

 better protected than most stems by virtue of their position, stems 



Fig. 10. — Diagram of a mint stem showing mechanical tissue at the angles. 

 M, mechanical tissue; C, cortex; VC, vascular tissue; HC, hollow center; 

 P, pith. 



are better protected structurally than any other plant parts except 

 seeds. The most important protective tissues of stems are the epi- 

 dermis in herbaceous and young woody stems and cork in older 

 stems. The epidermis ordinarily consists of a single layer of cells 

 the outer walls of which are water-proofed by a deposit of cutin 

 making up what is called the cuticle. Cutin is probably never de- 

 posited except on walls that are exposed to the air. The presence of 

 air, however, is most likely an indirect factor, the direct factor being 

 evaporation or desiccation since the cuticle is most strongly de- 

 veloped where there is free exposure in dry situations. 



In perennial stems the epidermis usually becomes ruptured during 

 the second or third years and its function is taken over by a complex 

 of tissues commonly called bark. Bark consists largely of phloem 

 and cork and it is the cork that makes it an efficient protective layer. 

 Cork is a tissue the cell walls of which have been water-proofed 



