ADAPTATIONS FOR REDUCING TRANSPIRATION. 391 



In most of the grasses, on the other hand, the stomata are most 

 numerous on the ventral or upper face, e. g., Ammophila arenaria, 

 Panicum amarum, Uniola paniculata, Spartina patens. But here 

 they are doubly or triply protected by the involution of the leaf 

 blades, by their position in furrows, and often by the presence of 

 stout hairs lining the wall of the furrows. Furthermore, the ax>proxi- 

 mately vertical position assumed by such involute leaves diminishes 

 the angle at which they are encountered by the light rays. 



(c) The liiestonie or fibro-vascular bundles of the stems and leaves 

 in many strand plants are more or less completely enveloped by mas- 

 sive groups of very thick-walled stereome. This undoubtedly serves 

 in great degree to protect the ascending and descending fluids from 

 evaporation. 



(5) Succulency. As was stated in the discussion of the Salt Marsh 

 formation, succulency is largely due to a strong development in the 

 interior of the organ, whether stem or leaf, of thin-walled, often 

 colorless parenchyma, which is believed to perform the function of a 

 water-storage tissue. This modification is especially characteristic of 

 desert plants, notably Cactaceae, but is also not infrequent among 

 strand plants, especially in the salt marshes. 1 Succulent species of 

 the sand strand are either — 



(a) Stem succulents, with leaves much reduced and a partial or 

 complete assumption by the stem of the functions of transpiration and 

 assimilation, e. g., Opuntia opuntia, or— 



(b) Leaf succulents, with well-developed, functionally active, 

 fleshy leaves, but often exhibiting, at the same time, some degree of 

 succulency in the stem, e. g., Cakile edentula, Tva imbricata, Euphor- 

 bia polyyonifolia, and, to a minor extent, Yucca jilamentosa (bases of 

 the leaves). 



It is well known that fleshy plants, while holding a larger supply of 

 water in their tissues than do uonsueculent species, also give up 

 their water less readily, and are therefore excellently adapted against 

 excessive transpiration. 



(G) Excretion of aromatic, volatile oils. It has been suggested 2 

 that plants which excrete essential oils are thereby protected to some 

 extent against loss of water. By evaporation of these oils an envelope 

 of aromatic air is formed about the plant, which, according to Tyndall, 



1 Massart (Mem. Soc. Roy. Bot. tie Belgiqtie, 32, pt. 1, p. IS) notes that succulents 

 are much more frequent in the salt marshes than on the sand strand of Belgium. 

 This he attributes to the effect of the strong, sand-liiden winds that blow over the 

 beach and dunes and riddle the soft, unprotected tissues of fleshy plants. He 

 remarks that on the sea-cliffs of Normandy succulent plants, being less exposed 

 to this danger, are more numerous. 



* Haberlandt, Physiologische Pflanzenanatomio, p. 325, ed. 2, p. 436; Volkens, 

 Flora der iigypt.-arab. Wuste, p. 4(5; Warming, Okolog. Pflanzengeog., p. 195. 



235<J2— No. 0—01 



