LEAVES 



633 



not so much with increased water accumulation as with the distribution of the water- 

 containing cells. Probably a leaf appears succulent, less because of the large 

 amount of water it contains than because its thickening at the expense of ex- 

 pansion concentrates the water within a compact region ; and leaf compactness in 

 contrast to expansion, already has been seen to be connected largely with high 

 relative transpiration (p. 598), and thus with cell-sap concentration. 



Edema. An explanation of leaf succulence may be suggested by a consideration 

 of edema, a phenomenon occasionally witnessed when turgor pressure is high and 

 transpiration low, and evidenced externally by the appearance over the leaf sur- 

 face of whitish emergences, known as intumescences (figs. 929, 930). Intumescences 

 develop on the leaves of Hibiscus vitifolius, Solanum tuberosum, and various other 



FIGS. 929, 930. Intumescences produced on cauliflower leaves (Brassica oleracea) by 

 chemical stimulation, the leaves having been sprayed with copper ammonium carbonate; 



929, a small portion of the lower leaf surface, five days after spraying ; i, intumescences: 



930, a cross section through an intumescence, highly magnified ; note the greatly hyper- 

 trophied mesophyll cells (h), which have broken through the lower epidermis () From 

 VON SCHRENK (929 drawn from a photographic reproduction). 



plants in moist chambers, and also on isolated leaves of Populus and Eucalyptus 

 and on the inner surfaces of pea pods that are placed in water ; in the tomato they 

 have been induced by forcing water into cut stems and by heating the soil in which 

 they grow. In all these cases a surplus of water in the plant causes the hypertrophy 

 of the leaf tissues, which, for lack of space within, are forced to break through the 

 epidermis as intumescences. Probably " water lenticels " (p. 663), the " substitute 

 hydathodes " of Conocephalus (p. 622), and the " breathing roots " of Jussiaea 

 (p. 508) are essentially identical with intumescences, not only having a similar-aspect, 

 but also developing under similar conditions. In some cases intumescences are 

 developed also by chemical stimulation, as in the leaves of the cauliflower (figs. 



929, 93o). 



Succulence and intumescence. While succulent and intumescent leaves seem to 

 have certain superficial resemblances, each being very juicy and having cells filled 

 almost to bursting with cell sap, they differ in most essential respects. Intumescence 



