342 PHYSIOLOGY 



water must pass osmotically. This could be accomplished to a certain 

 extent by elongating the cells in the main direction of travel; and it may 

 be that elongation of the cells was one of the early steps in the evolution 

 of a conducting system. To-day there are plants in which such strands 

 exist, as in the stalk of the sporophyte of liverworts and mosses, and 

 these are often accounted rudimentary conducting tissues.^ 



Lignified tracheids. — The complete elimination of the cytoplasmic 

 membranes may well have been a second step in evolution. This would 

 make movement more easy by removing just so much resistance from 

 the path. If in addition the walls were altered so as to be more freely 

 permeable to water, movement would thus be further facilitated. That 

 change, known as lignification, is indeed common. Then by thickening 

 the wall only in parts, leaving the rest thin, passage of water through it by 

 way of the thinner areas would be still easier. Strands of elongated cells 

 of this sort constitute the endings of the conducting system in the leaves 

 of almost all plants, and they form almost the whole of the characteristic 

 wood in gymnosperms and the conducting strands in pteridophytes. 



Tracheae. — One further step attains the condition in the most per- 

 fectly developed conducting tissues, namely, the resorption of the greater 

 number of the transverse partition walls between the elepients, forming 

 cell fusions of great length, known as ducts, or vessels, or tracheae, the 

 latter from their occasional resemblance to the human trachea and the 

 air tubes of insects. Resorption does not usually occur near the endings 

 of the strands in the leaves; and in gymnosperms it fails except in the 

 primary strands. But the other changes do occur, and the elements 

 being cells and not cell fusions are distinguished as tracheids. Following 

 the history of any row of cells which is to become a duct, there is first the 

 elongation of the cells; then the unequal thickening of the wall and its 

 lignification, together with the resorption of most of the end walls; and 

 finally the disappearance of the protoplasm. Some such steps as these 

 may also have marked the evolution of the conducting system through 

 earlier ages. 



Xylem. — The conducting system in the larger plants now consists 

 of a series of strands known as xylem strands or as the xylem regions of 

 the vascular bundles (p. 242). Physiologically it is more satisfactory to 

 treat the xylem as independent of the phloem (p. 242), for although they 

 are usually closely associated in their course, they may be independent, 

 and the functions of the two are quite unlike. The xylem strands form a 

 ' This is based too much on analogy and inference; the experimental evidence is weak. 



