126 



THE PLANT 



7. The succulent form. Plants with succulent stems such as the Cac^ 

 taceae, Stapelia, and Euphorbia have not only decreased water loss by ex- 

 treme reduction or loss of the leaves, and the reduction of stem surface, but 

 they also offset transpiration by means of storage tissues containing- a mu- 

 cilaginous sap. The cuticle is usually highly developed and the stomata 

 sunken. Thorns and spines are also more or less characteristic features. 



170 Bog plants. Many of the xerophytic types just described are found 

 in ponds, bogs, and swamps, where the water supply is excessive, and hydro- 

 phytes would be expected. The explanation 

 that "swamp xerophytes" are due to the 

 presence of humic acids which inhibit absorp- 

 tion and aeration in the roots has been 

 generally accepted. As Schimper has ex- 

 pressed it, bogs and swamps are "physiologi- 

 cally dry", i. e., the available water is small 

 in amount, in spite of the great total water- 

 content. Burgerstein (/. c, 142) has shown, 

 however, that maize plants transpire, i. e., 

 absorb, three times as much water in a solu- 

 tion of 0.5 per cent of oxalic acid as they do 

 in distilled water, and that branches of Taxus 

 in a solution containing i per cent of tartaric 

 acid absorb more than twice as much as in 

 distilled water. Consequently, it seems im- 

 probable that small quantities of humic acids 

 should decrease absorption to the extent 

 necessary for the production of xerophytes in 

 ponds and bogs. Indeed, in many ponds and 

 streams, where Helcocharis, Scirpus, Juncns, 

 etc., grow, not a trace of acid is discoverable. 

 Furthermore, plants with a characteristic 

 hydrophytic structure throughout, such as 

 Ranunculus, Caltha, Liidivigia, Sagittaria, 

 etc., are regularly found growing alongside of apparent xerophytes. Many 

 of the latter, furthermore, show a striking contrast in size and vigor of 

 growth in places where they grow both upon dry gravel banks and in the 

 water, indicating that the available water-content is much greater in the latter. 

 Finally, many so-called "swamp xerophytes" possess typically hydrophytic 

 structures, such as air-passages, diaphragms, etc. In spite of a growing 

 feeling that the xerophytic features of certain amphibious plants can not 

 be ascribed to a low chresard in ponds and swamps, a satisfactory explana- 



Fig. 35. Polygonum bistor- 

 toides, a stable type: 1, meso- 

 phyll (chresard, 25^); 2, xerophyll 

 (chresard, Z-b%). X 130. 



