DEPARTMENT OF BOTANICAL RESEARCH. 85 



are expressions of the response of the species to the distinctive aeration 

 and temperature conditions of the soil environment. 



The Origin and Physical Basis of Succulence in Plants, by D. T. MacDougal and 



H. A. Spoehr. 



Two types of succulent or fleshy plants are recognized by the bot- 

 anist — those which like the cacti grow in dry regions, and those of the 

 seashore, sometimes known as halophytes. The chief anatomical 

 characteristic of these forms consists in the fact that tracts of thin- 

 walled cells have become enlarged either by the multiplication of the 

 number of cells or by the exaggerated growth of the original number, 

 and LQ these cell-masses relatively large quantities of water or cell- 

 sap are held. The origin and evolutionary development of this type 

 of vegetation has been the subject of much speculation. Nearly all 

 succulent plants are characterized by marked acidity of the tissues, 

 and as many of the species inhabit localities in which the soil is highly 

 charged with salts, these physical factors have been made to account 

 for the accumulation of water in plants through the play of osmotic 

 forces. All explanations of this character have, however, been found 

 inadequate. 



The discovery by Professor H. M. Richards of the fact that the 

 Castilleja and the Erigeron, native to the region about the Coastal 

 Laboratory, each included some indi"viduals which were thin-leafed 

 and others which were thick-leafed, and that these leaf characteristics 

 were correlated with the water-supply of the plants, has furnished us 

 the material upon which an explanation of succulence might be based. 



The measurements of hydration or imbibition by plant tissues, 

 which are discussed in several parts of this report, make it appear 

 that the protoplasm of plants consists largely of pentosans, of which 

 the mucilages are an example, mixed with a smaller proportion of pro- 

 tein derivatives, and some salts. Naturally, since succulence implies 

 an increased storage of water, attention was turned to this water- 

 holding mechanism of the cell in search of an explanation of its 

 increased capacity. 



The fundamental fact has been established, in series of analyses of 

 the carbohydrates of desert plants, that when cells containing poly- 

 saccharides undergo a depletion of the water-content, these sugars 

 are reduced to the pentosans, of which the mucilages are largely com- 

 posed. The imbibition capacity of the polysaccharides is smaU. 

 Their transformation from this form into that of the pentosans gives 

 the increased capacity characteristic of the pentosans, so that with- 

 out any addition of material to a cell, but simply by the loss of water, 

 a change takes place by which the cell is capable of absorbing and 

 holding vastly greater proportions of water. In other words, aridity 

 by causing undue loss of water induces a change in the ceU by which 



