CHAP. XXXII STORAGE OF WATER BY LAND-PLANTS 123 



i. Succulent-stemmed (Chylocaulous) Plants. 



In these plants the stem is fleshy and jiiic}'. The leaves are suppressed 

 in the most marked types, or they are reduced to scales or thorns ; the 

 stem has assumed the assimilatory functions of foliage, and the trans- 

 piring surface is thereby greatly reduced. 



The most common and extreme types are Cactaceae in America, 

 Stapelia in South Africa, and species of Euphorbia which occur mainly 

 in Africa. To these may be added the geraniaceous Sarcocaulon in 

 South Africa. 



In the various genera there occur a series of shapes whose efficiency 

 has been demonstrated by Gobel,^ Noll,- and others. Frequent among 

 such shapes are those like the sphere, prism, or cylinder, that combine 

 smallness of surface with largeness of volume. This is advantageous 

 in relation to storage of water, but disadvantageous in regard to assimila- 

 tion. One stage towards increase of surface and therefore of assimilation 

 is represented by the production of ridges, processes, bosses and the like, 

 in Mammillaria, Echinopsis, and other Cactaceae. These protuberances 

 are set vertically, that is, in a manner which does not render them so 

 easily heated by the sun's rays and this is of advantage as their internal 

 temperature is often high.^ 



Here may be included pseudo-bulbs which occur mainly in epiphytic 

 orchids ; they are tuberous green stems, consisting of one or more seg- 

 ments (thus bearing one or more leaves) ; they persist long after the 

 leaves have fallen, serving as water-reservoirs and often containing 

 mucilaginous sap. 



ii. Succulent-leaved (Chylophyllous) Plants. 



In plants with succulent leaves the stem is normal in form, except 

 that its internodes are often short and its leaves consequent!}' arranged 

 in rosettes. The leaves are thick, stumpy, sessile, usually elongate and 

 narrow, often cylindrical (if we except the sphere, a prism or cylinder 

 has the smallest surface in relation to volume) ; they often are continued 

 at the margins or apex into thorns, but apart from this are usually un- 

 divided and entire. As examples of plants having their leaves in rosettes 

 we may mention Agave, Aloe, Sempervivum, Echeveria, species of 

 Mesembryanthemum, and some epiphytic orchids ; elongated internodes 

 are developed by Sedum, BryophyUum, Portulaca, and Senecio (Kleinia). 



Succulent-stemmed and succulent-leaved plants are both represented 

 among haloph}i:es. 



Succulent plants deviate from other chlorophyll-possessing plants 

 in both respiration and assimilation. The divers structural features 

 that obstruct transpiration at the same time constitute an obstacle to 

 the assimilation of carbon dioxide ; at night-time, during respiration, 

 there is produced only little carbon dioxide but much malic or other 

 organic acid, which is utilized in the manufacture of carbohydrates on 

 the following day.'* 



' Gobel, 1889-93. ' ^'oll' '893- 



^ Concerning the morphology of Cactaceae, see Vochting. 1874, 1894; Gobel, 

 loc. cit. * Aubert, 1892; Jost, 1903, Lecture 15. 



