BASIS OF SUCCULENCE IN PLANTS 



D. T. MacDougal, H. M. Richards, and H. A. Spoehr 



Succulents may be characterized as plants in which the paren- 

 ch>Tnatous elements show an exaggerated development with rela- 

 tion to the more rigid tissues, and, unlike pith or medullary tracts, 

 the masses of thin-walled cells remain distended and turgid. The 

 liquid contents of such cells may or may not contain much dissolved 

 material. The disposition of the water-holding tracts varies from 

 leaves to stems and roots, but in all cases the most important general 

 effect is one of massiveness, and the surfaces of succulent plants 

 may in such forms as the barrel cacti bear the smallest possible 

 proportion to the mass, that of a globe. 



The ecologist recognizes two general types of succulents, those 

 of the arid regions, which are of a xerophytic character, exemphfied 

 by the cacti; and the halophytes or fleshy seashore plants, also at 

 home in alkaUne areas. The plants of the two types are quite 

 unlike in their transpiratory relations. The desert succulents may 

 lose water so slowly that an existence of several years may be main- 

 tained upon the water in the thin-walled tracts.' On the other 

 hand, the halophytes or fleshy shore plants may flag and wilt as 

 readily as any thin-leaved form, due to the rapid loss of water from 

 the surfaces. The origination of these striking forms has been the 

 subject of much speculation, but all attempts to connect suc- 

 culency in a causal way with the presence of salts in the soil, or in 

 the plant with the well known high acidity of many of these forms, 

 or with any purposeful development of water storage capacity, 

 have been inadequate. 



Our concurrent observations and experiments may be briefly 

 summarized as follows: 



I. A Castilleja native to the region about the Coastal Labora- 

 tory, at Carmel, California, includes two habitat forms, geneticaUy 

 identical, one with thin leaves growing in the open forest formation, 



' MacDougal, D. T., et al., End results of desiccation and respiration in succulent 

 plants. Physiol. Researches i : 2 89-3 25. 1 9 1 5 . 



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