118 PLANT PHYSIOLOGY 



in the direction of least resistance, that is, from hviiig and 

 turgid cells into empty tube-like tracheids and ducts. At 

 any point in the plant the adjacent parenchyma cells may 

 absorb water from the vascular tissues just as the root- 

 hairs absorb water from the soil, and by the same physical 

 means. Whether there are continuous columns of water in 

 the ducts or not, there is a continuous body of water in the 

 walls of the ducts, and so the withdrawal of water at any 

 point will induce a movement of water toward that point 

 from parts better supplied. It is ordinarily from below that 

 water is drawn, for ordinarily the roothairs supply the 

 needs of the whole plant, but this is not necessarily the case, 

 for through the conducting tissues water will pass up or 

 down according to circumstances. 



The vascular tissues form a continuous system, often 

 much complicated in arrangement but proportionally in- 

 creased in usefulness, of water-conveying cells and vessels 

 extending from base to tip of the plant. At frequent inter- 

 vals the bundles, which run more or less distinct from one 

 another, anastomose and thus combine the vascular tissues 

 into one effective system. Branches are given off from the 

 main channels, so that buds, leaves, branches, even hairs 

 ( e. g. glandular hairs of Drosera ) , are reached by the con- 

 ducting system. By this means all the parenchyma cells, 

 which are the actively living cells of the plant-body, are 

 supplied directly or indirectly. The amount of water and of 

 dissolved matters supplied to any part will depend upon the 

 demand, upon the amount lost and consumed. For ex- 

 ample, two adjacent vascular bundles, running to two 

 leaves, will convey different volumes of solutions if from the 

 one leaf more water evaporates than from the other, or if 

 in one leaf more water and dissolved matters are used than 

 in the other. We thus see that the amounts transferred 

 through parts, and consequently through the whole, of the 

 vascular system are dependent upon the activities of living 

 cells : tirst, upon those living cells which absorb nutrient 

 8olu1;ions from the soil and from which other living cells 

 osmotically absorb them, the cells abutting upon ducts 

 and tracheids discharging the excess of water into these 



