CLAWSON: ANATOMY OF JANUSIA. 195 



are found where the trichome rests on a stalk and is fastened 

 to the epidermis at a point where the cells which surround this 

 stalk are raised above the surface of the leaf. 



It may be that this hair has something to do with the ab- 

 sorption of water, which has to be used with so great economy 

 by the desert plants. Haberlandt describes a hair of Diplotaxis 

 harra that is used for such a purpose. It is cutinized, and at 

 the base has pits through which water may pass from the sur- 

 face of the hair to the inner pai't of the hair, and then to the 

 epidermal cells and on to the mesophyll. Since the two-armed 

 trichome is so characteristic of the Malpighiacefe — so char- 

 acteristic that they are called Malpighian hairs — it would not 

 seem at all improbable that the plant would modify one of its 

 most common structures in adapting itself for desert condi- 

 tions. It may be possible that during the night, when the hair 

 would give off heat enough by radiation to bring the tempera- 

 ture of the air to the point of saturation, water would be 

 collected into the cellulose hair; from this the water would 

 pass by filtration or osmosis into the cavity of the pore under 

 the membrane through that part of the stalk of the trichome 

 which has cellulose walls (fig. 29, C). As the cavity would fill 

 up, the membrane would be forced outward, lifting with it the 

 hair, thus opening the pore beneath. The plug would fall into 

 position again as soon as transpiration reduced the pressure 

 from below the membrane. The cutinized end of the plug 

 would then retard further loss of water at this point. This 

 explanation of the function of the hair, at its best, is only 

 theoretical. No fresh material was at hand for physiological 

 experiments. Several difficulties arise in applying this theory 

 of the function of the hair. Would water pass through a cel- 

 lulose cell wall and fill the cell cavity if the cell were dead? 

 Whether the cell contained protoplasm at the time of collection 

 was not determined. It would be easy enough for the water to 

 pass from the outside of the hair to its inside, provided the cell 

 was living and filled with protoplasm, but the question then 

 arises, even if the cell were alive, how could it live in the desert 

 with only cellulose walls to keep it from drying up? It would 

 be interesting for one having fresh material to work out the 

 ontogeny and physiology of these hairs. 



In the Dilleniacese simple unicellular and multicellular tri- 

 chomes may be seen loose at each end and fastened at the 



a-Univ. Sci. Bull.. Vol. VII. No. 8. 



