380 LOUIS MAX HICKERNELL. 



present without producing death by desiccation. Now under 

 normal conditions about 95 per cent, of the weight of the cactus 

 is made up of water. This would mean that a decrease of 

 approximately one half in the weight of the plant as a result of 

 drying, is usually fatal. In Philodina the minuteness of the 

 animal, together with the fact that it normally lives in water, 

 makes it almost impossible to determine the changes in weight 

 during the drying period. It is easy, however, to follow the 

 changes in volume which occur during drying and anyone can 

 see if he will dry a Philodina upon a slide under a microscope 

 that the animal decreases to from one third to one fourth of its 

 original size. Since most of the weight of the rotifer body, 

 certainly more than 95 per cent., is made up of water it will 

 be seen at once that the percentage of substance lost by the 

 rotifer during desiccation is much higher than that observed in 

 the cactus. 



The desiccation process in the cactus caused few changes in 

 the thickness of cell-walls. In the plant treated for seventy- 

 three months the cuticle was slightly thicker than normal while 

 the outer walls of the epidermal cells were thinner. This result 

 was obtained in the most severe test of the series. In my experi- 

 ments on Philodina the most severe drying caused absolutely 

 no thickening of the cell walls which bordered the surface, but 

 on the contrary by a loss of water from the hypodermal layer an 

 actual decrease in the thickness of the integument was observed 

 in all cases. 



Perhaps the most pronounced cytological effects of desiccation 

 are those recorded for the cortex cells of Echinocactus. Here 

 there was an entire disappearance of the protoplasts and a 

 hydrolysis of the cell walls. There is nothing in my experiments 

 upon Philodina which parallels this. The changes described in 

 the stomach tissue are similar but in no case have I ever observed 

 the disappearance of cell-walls or the formation of spaces as a 

 result of the disintegration of cell-elements. 



That the different changes during the desiccation process and 

 subsequent recovery in Echinocactus are very slow is well shown 

 by McDougal and Brown when they state that a plant which 

 had been desiccated for forty-two months and was then placed 

 under normal conditions in the soil for twenty-two months did 



