374 The American Naturalist [May, 



oles and ramifying in the veins of the leaves. It seems to 

 follow from this that whatever be the path of the water cur- 

 rent in the stem itself, it can enter the body of the plant in 

 quantities sufficient for transpiration purposes only along the 

 pathway of the spirals, and can reach the leaves only through 

 the same channels. 



The pitted vessels are probably sometimes nearly full of 

 water, and at other times nearly empty, the amount depend- 

 ing on the quantity in the soil and on the activity of transpi- 

 ration. Owing to the number of very thin places or actual 

 perforations in their walls, they undoubtedly contain air at all 

 times and probably often in large quantities. I regard these 

 vessels as water reservoirs. In this capacity they appear to 

 be admirably adapted to serve the needs of a class of plants 

 which (on account of the extent and unprotected nature of 

 their transpiring surface) often make sudden and very large 

 demands on the stem for water, — demands greater than can 

 be met by the immediate activity of the roots. There is, how- 

 ever, nothing against the supposition that when they are not 

 full of water, they may also serve as aerating organs, the 

 stems being alive and chlorophyll-bearing clear to the cen- 

 ter. The function of the spiral vessels, according to my con- 

 ception, is quite different. They also contain a greater or lesser 

 quantity of water, according to the activity of transpira- 

 tion and the amount procurable from the soil or from the 

 neighboring reservoirs (the pitted vessels), but unlike the pitted 

 vessels, they are surrounded by a living, non-lignified, non- 

 lacunose parenchyma, and there is no free access of air to 

 their interior, but, on the contrary, so far as we can judge 

 from the anatomical structure, this part of the plant has been 

 developed with special reference to keeping it out. When the 

 spirals are not full of water, they probably contain rarefied 

 air. The very thin walls of these spiral vessels bear on their 

 inner face lignified annular or spiral thickenings, which are 

 probably of great service in strengthening the delicate walls, 

 so that they may be strong enough to resist the collapsing 

 tendency of the vacuum pull due to the osmotic pressure, and 

 yet remain thin enough to readily allow water to filter into 



