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. 1 
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 elements, 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 
1 This is based too much on analogy and inference; the experimental evidence is weak. 
