6o REPORTS OF INVESTIGATIONS AND PROJECTS. 



direct catalytic action of the reagents used, since the opinion grows in 

 strength that the salts used, when present in the plant, sustain chiefly a 

 catalyzing function in the cell. 



The Water-balance in Succulents (by Dr. D. T. MacDougal) : 



The observations on the variation and composition of the solutions accu- 

 mulated in the bodies of cacti and other desert plants have been brought to a 

 stage where the results seem worthy of presentation in a paper now being 

 completed. In all of the forms tested growth and reproduction are depend- 

 ent on the presence of the full capacity of the plant, but such plants may be 

 separated from an available supply for extended periods without injury. 

 The ratio of transpiration to the amount present is such that long periods are 

 necessary to deplete the balance to a point where the plant perishes from lack 

 of water. The absorbing capacity of the roots of such plants is far in excess 

 of their needs during any given period. The concentration of the dissolved 

 salts of course increases with the loss of water from cell-sap, but no marked 

 change in acidity ensues. Plants have been deprived of over 70 per cent 

 of their water-balance without serious injury, the depleting process being 

 extended over many months or years in some tests. 



Specialized desert plants are of two types, spinose forms and succulents. 

 The absence of these forms from the fossil records justifies the conclusion 

 that the cacti and other xerophytes have originated since the beginning of the 

 Pleistocene. The earliest step in the action by which xerophytic plants were 

 produced would consist in the reduction of the members of the shoot from 

 which thorny or spinose plants would be produced. A second step, which 

 might be in progress at the same time as the first, would consist in the 

 enlargement of tissues containing water producing the succulents. Such 

 plants with a large water-balance therefore represent the most advanced 

 stage of vegetative development in the movement by which vegetation has 

 gradually occupied the dry land and as a very recent development has invaded 

 the desert. 



Origination of Parasitism (by Dr. D. T. MacDougal) : 



A consideration of the general relations of the large water-balance carried 

 by succulents led to the suggestion that this material might offer conditions 

 highly favorable to the development of parasitism among the seed-plants. 

 After much preliminary experimentation it has been found that healed or 

 regenerated cuttings of species which propagate readily by this means might 

 continue existence and make growth when the emerging roots were allowed to 

 penetrate the tissues of another living plant as a substratum, as announced in 

 the report for 1908. Such cases of xe no- parasitism may be more or less 

 permanent, as some of the preparations made in 1908 are still in existence. 

 The results in question are to be sharply distinguished from grafts, in which 



