DESICCATION IN PHILODINA ROSEOLA. 379 



in volume in the plants subjected to less severe conditions. 

 The nuclei decreased to about half size. The cell-walls thickened 

 enormously in the extreme cases of drying and in many cases 

 never recovered normal thickness. 



The results of McDougal and Brown are most interesting 

 when compared with my own. The observations upon the 

 cytoplasmic and nuclear changes in the dried tissues show that 

 they are similar in many respects with those in corn embryos and 

 rotifer tissues described in my earlier paper. 



The length of time during which an Echinocactus may survive 

 in the open at the expense of its surplus food material and water 

 was found to be no more than two years while similar plants in 

 diffuse light were sound after six years of starvation. The cac- 

 tus, then, which normally lives in an environment containing 

 little moisture is less able to adapt itself to extreme dry con- 

 ditions than is the rotifer which normally swims about in water. 

 The extreme length of time during which a Philodina may remain 

 dry is not known but well authenticated records show that they 

 have been known to withstand a period of dryness for as much 

 as twenty-seven years. To date no experimental data are 

 recorded to show the effects of varying intensities of light upon 

 the time during which rotifers may live in a desiccated condition. 

 This point I hope to treat in a future publication. 



Another result of interest in the cactus experiments is that in 

 extended desiccation and starvation the plasmatic colloids are 

 eventually broken down by katabolic action. This katabolic 

 activity includes hydrolysis of the cell-walls of the cortex. Now 

 in Philodina katabolic activity undoubtedly takes place during 

 the dry periods. This was most clearly seen in the tissue of the 

 stomach-intestine. But while the reserve food granules dis- 

 appear, no change is observable in the walls of any cell. It 

 might be said from the nature of the two cases that the reaction 

 in the cactus is irreversible while in the rotifer it is reversible, 

 for the cells of the cactus perhaps never assume a perfectly 

 normal condition after the drying process while a sufficient food 

 supply is all that is necessary to make the rotifer quickly resume 

 its normal structure. 



McDougal found that in the cactus the loss in weight in full 

 illumination may not greatly exceed 50 per cent, of the water 



