DEPARTMENT OF BOTANICAL RESEARCH. 73 



A second feature, the force of expansion of the invading protoplasts, 

 would be no less important. The pressure set up, like that of a swelling 

 seed,would be great enough to cause mechanical penetration of the host, 

 as it would be far greater than any force attributable to osmotic action. 



After the haustorial development has carried that organ to a mature 

 stage the nutritive contact with the host is one in which osmosis doubt- 

 less plays an important part. The proportion of nitrogenous sub- 

 stance in the parasite or the acidity of the concentration of salts might 

 be the determining factors in both the making and maintenance of a 

 nutritive couple of host and parasite. (See MacDougal and Cannon, 

 Conditions of Parasitism in Plants, Carnegie Inst. Wash. Pub. No. 

 129, and MacDougal, The Beginnings and Physical Basis of Parasit- 

 ism, Plant World, August 1917.) 



PHOTOSYNTHESIS, METABOLISM. AND GROWTH. 

 The Carbohydrate Economy of Cacti, hy H. A. Spoehr. 



The investigations on the carbohydrate metabolism of the cacti have 

 been continued, repeating with improved methods and extending the 

 previous studies, especially in regard to the seasonal variations in the 

 carbohydrate balance. Owing to the fact that the material used (the 

 platyopuntias and Opuntia versicolor) lends itself so admirably to 

 experimentation, it has been possible to gain an insight into various 

 phases of the carbohydrate economy of plants which as yet has not 

 been possible from work with thin leaves. 



The pui*pose of this work, begun two years ago, is primarily to 

 gather data and general facts which can be brought to converge for 

 an attack on the problems of photosynthesis. Preliminary experi- 

 ments, carried out several years ago, indicated clearly that prerequisite 

 to a rational approach on the problem of the manner in which sugars 

 are formed in the chlorophyllous leaf is a clearer understanding of the 

 conditions governing the equilibria and mutual transformations of the 

 various groups of carbohydrates in the leaf, as well as of the fate of 

 these substances in the general metabolism. 



From these studies it becomes evident that the amount (or the 

 proportion to the total) of certain sugars present in a leaf after insola- 

 tion can not be taken as an indication of the rate at which these sugars 

 are formed in the photosynthetic process, as has been almost univer- 

 sally done, for under varying conditions of water-content and tem- 

 perature, such as occur in a leaf in the sunlight, there is a consequent 

 shifting of the carbohydrate equilibrium, resulting in the removal of 

 one or the accumulation of another group of sugars according to cir- 

 cumstances. Therefore, these conditions (e. g., water-content and 

 temperature) either must be kept constant or, what is more feasible, 

 the equilibrium under the particular circumstances must be estabhshed 



