28 THE PHYSIOLOGY OF PLANTS 



funnel, and the pressure is spent on the thickened portion, 

 ■which will resist any pressure likely to occur under such 

 conditions. 



This useful system of valves is only developed between two 

 water -conducting elements ; if, however, a vessel adjoins a 

 cell containing protoplasm or reserve substance, the bordered 

 pit will only be developed on one side. If the parenchy- 

 matous cells are permanently rich in water, while the vessel 

 is poor in water, or even filled with air, then the high pressure 

 (turgidity) of the parenchymatous cells will cause the cell-wall 

 to bulge out through the pit into the lumen of the vessel, and 

 the latter will become filled by protrusions on all sides from 

 the adjoining cells. These become divided, off from the 

 original cells which gave rise to them by transverse walls, and 

 enlarge so as to completely plug up the vessel. This filling 

 of the vessels with bladder-like cell protrusions (thylloses) 

 takes place especially when the wood system becomes damaged, 

 and it affords a protection of the injured portion against the 

 effects of injurious atmospheric conditions. 



Let us now sum up shortly the results of this paragraph. 

 The vessels and vessel-like cells, all tracheal elements there- 

 fore, are at the commencement filled with water, but can be 

 partially and temporarily emptied by the transpiration from 

 the leafy portion of the plant. In that case they become 

 filled again by the pressure of water from the parenchymatous 

 cells surrounding them. If the vessels become emptied of 

 water, the air within them must take its place, and fill the 

 cell or vessel. In doing this it becomes greatly rarified ; and if 

 it be cut under water, it will suck up the latter with great 

 avidity (negative pressure). Besides this rising of the water 

 by suction, an upward forcing takes place by root pressure. 

 Lastly, the capillarity of the vessels, and the changes effected 

 in the Jamin's chains which they contain, are of considerable 

 importance. These two physical forces make a continuous flow 

 of water in the vessels possible, if the leaf structures only use 

 up a moderate amount of water compared with the amount 

 absorbed. They also enable a passage of water to take 

 place when the vascular system is considerably emptied 

 by an inordinate amount of transpiration. The regulation of 



