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 (J. 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 Heleocharis, Scirpus, Juncus, 
Fig. 35. Polygonum bistor- etc., grow, not a trace of acid is discoverable. 
ee oe sdsontell Furthermore, plants with a characteristic 
(chresard, 3-52). 180. hydrophytic structure throughout, such as 
Ranunculus, Caltha, Ludwigia, Sagittaria; 
ete., 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- 
