696 



ECOLOGY 



chyma. Tyloses develop also at wounded surfaces and in the vascular tracts of 

 certain plants attacked by parasites; in these cases they clearly are advantageous 

 in checking conduction, but no particular advantage or disadvantage is apparent 

 in the clogging of duramen tissues. 



The rSle of the pericycle and of the endodermis. 

 — The chief role of the pericycle, which is a 

 meristematic layer, is the origination of new 

 lateral organs, especially roots. The endodermis 

 often is a protective layer, particularly in roots 

 and rhizomes, its suberized walls serving to 

 check local losses of water from the bundle. 

 In aerial stems it may serve as a region of 

 L(^ "^Y{/f ^^\i'r~'=^S^ I starch accumulation, whence the name starch 



W sheath. 



4. MECHANICAL TISSUES 



Introductory remarks. — In small land 

 plants and in water plants of all sizes 

 mechanical tissues are poorly if at all 

 developed, their rigidity when present 

 being due to the cell walls and to 

 turgor. Large aerial and soil organs, 

 however, are subjected to considerable 

 strain, and conspicuous mechanical 

 tissues commonly are present, insuring 

 the maintenance of form and position. 



The mechanical elements or stereids. 

 — Bast and wood fibers. — The most 

 representative mechanical elements are 

 the bast fibers, which are especially char- 

 acteristic of the phloem. They are elon- 

 gated prosenchymatic structures with 

 attenuated points, and thus have long 

 regions of contact with adjoining fibers 

 in the same longitudinal row, insuring 

 a degree of dovetailing perhaps un- 

 equaled elsewhere (fig. 1020, B). The 

 fibers commonly average one or two 

 millimeters in length, but in ramie 

 (Boehmeria), one of the most important 

 fiber plants, the length may be two 



Fig. 1020. — Bast fibers of the 

 century plant (Agave americana): 

 A, a cross section through a bundle 

 of bast fibers, showing the uniform 

 stratification of the walls (w), and 

 the small size of the lamina (6) in 

 proportion to their thickness; 5, a. 

 longitudinal section through such a 

 bundle, showing prosenchymatous 

 cell ends (p) dove-tailing with one 

 another; s, spirally arranged wall 

 slits ; other lettering as in .4 ; highly 

 magnified. 



