102 



Hydration and Growth. 



The cell-sap of Echinocactus grown at the Desert Laboratory has a 

 low content of solid matter, and shows osmotic pressures of 3 to 5 

 atmospheres, calculated by cryoscopic methods. The acidity of the 

 massive body is greatest in the external layers and decreases toward the 

 center, and also shows daily variation which is greatest in the exter- 

 nal layer. This is illustrated by the data in table 81, obtained by 

 Mr. E. R. Long, in which A designates the external layer and D 

 the innermost parenchyma, B and C being intermediate.^ 



Table 81. 



The freshly expressed juice of regions C and D of such a plant at 

 midday caused sections of a biocolloid consisting of 5 parts agar, 3 

 parts mucilage of Opuntia, and 1 each of gelatine and bean protein to 

 swell about 840 per cent at 20° C, while similar sections increased 

 about 2,500 per cent in distilled water. Dried slices of the paren- 

 chymatous tissue similar to that from which the juice was expressed 

 swelled 115 per cent in the juice, while they increased but 42 per cent 

 in distilled water at the same temperature. 



Some of the sap of Echinocactus caused dried slices of Opuntia, the 

 swelling of which has been described in detail in Chapter VII, to swell 

 372 per cent at 20° C, which is to be compared with 550 per cent in 

 distilled water. It is notable, however, that such dried sections of 

 Opuntia increased 325 per cent in the sap expressed from living joints 

 of the same kind, thus swelling less in its own sap than in that of 

 Echinocactus. It seems probable that the possibilities of parasitism 

 might be more profitably sought in these hydration relations rather 

 than in the simpler osmotic coefficients to which the author attributed 

 great importance in his original study of this matter.^ 



It is highly probable that any cell-mass would absorb water and 

 swell in fresh sap from other cell-masses of the same kind in an equiva- 

 lent condition. Kunkel placed spores of Monilia sitophila (Mont) in 

 the sap of equivalent spores and obtained no plasmolytic effects and 

 did not measure for swelling. When the sap was reduced to one-tenth 



^ Long, E. R. Acid accumulation and destruction in large succulents. The Plant World, 18: 

 No. 10, 261. 1918. 



* MacDougal and Cannon. The conditions of parasitism in plants. Carnegie Inst. Wash. 

 Pub. No. 129, 1910. See also MacDougal, The beginnings and physical basis of parasitism. The 

 Plant World, 20: p. 238. 1917. 



