72 CARNEGIE INSTITUTION OF WASHINGTON. 



and accompany growth, running parallel to its course and being deter- 

 mined in greater part b}^ identical causes. 



That the water balance is actually decreased at night and increased 

 by day has been found by Mrs. E. B. Shreve, who says of a cylindro- 

 puntia:^ 



"It was found, under conditions of average transpiration, such as occur in 

 the greenhouse in summer, that the water intake at night is less than the 

 outgo, while during the day the intake is greater than or at least equal to the 

 outgo. . . . An examination of the water-content of stems from plants in the 

 open and from the greenhouse showed that the highest water-content is at 

 5 p. m. after the close of a bright day, and the lowest just before daylight the 

 next morning, with an intermediate amount at noon." 



The Effect of Desiccation on the Structure of Echinocactus ivislizeni, 



by J. G. Brown. 



The effect of desiccation on the structure of living tissues has been 

 studied chiefly by zoologists who were interested in the ability of the 

 rotifers and other invertebrates to survive extended periods of drought. 



The capacity of plants, such as certain algse, pteridophytes, and 

 liverworts, to endure periodic drying, is even more remarkable than 

 the ability of the rotifer to resist desiccation, but there is a paucity of 

 Hterature on the changes of a cytological character in the cells of these 

 plants resulting from conditions of drought. 



The object of the investigation here reported was to determine 

 changes in structure brought about by continued deprivation of water, 

 and to discover any evidence of recovery on the part of the plant when, 

 after a period of desiccation, it was again subjected to normal out-of- 

 door conditions. Prehminary to the study of the desiccated material 

 a careful examination of the structure of the normal plant was made. 

 The principal conclusions warranted by the facts obtained are as 

 follows : 



Extended desiccation and starvation made no alteration in the 

 integument of Echinocactus, but in a plant which had been thus treated 

 for 73 months the cuticle was thicker than normal, while the outer walls 

 of the epidermal cells were thinner. Cytoplasm and nuclei in the 

 epidermal system were reduced, but new cork layers were being formed 

 as in normal plants. Cell division was seen in the epidermal layer at 

 the bottom of the grooves of the stem. The stomata remained perma- 

 nently open and many were in a collapsed condition. Guard-cells of 

 stomata differed from the normal in having the anterior walls thinner 

 as compared with the posterior walls. 



The pahsade layer was thinner in desiccated than in normal plants 

 of Echinocactus. The cytoplasm was reduced to small masses in the 

 angles of the cells and the nuclei were variously deformed and reduced 



'Rept. Dcpt. Bot. Research, Carnegie Inst. Wash., Year Book for 1914, pp. 98, 99. 



