SUCCULENT PLANTS. 177 



bounded by two kidney-shaped cells containing 

 green matter, by the expansion and contraction of 

 which cells the aperture is diminished and in- 

 creased. These stomata are always placed over 

 the spaces between the cells of tissue, and so 

 minute and numerous are they that in some leaves 

 70,000 occur in a square inch of cuticle; the 

 longest are about the -^ of an inch in length, 

 and the least are not the ^-oW- Their office 

 is to allow of the passage of watery vapour and 

 gases from the soft tissues of the leaf, and to 

 permit the access of the sunlight and atmosphere 

 and their chemical influences to the sap or juice, 

 in order to its being converted into a substance 

 adapted to the nourishment of the plant. The 

 exhalation from the leaves of inland plants is so 

 great and rapid that, if planted for experiment 

 sake in the dry hot sand in which any sea-shore 

 plants thrive, they must speedily wither and dry 

 up. If, therefore, there were no modification of 

 structure in the leaves of the plants now in 

 question, they could not even for a few hours exist 

 in the situations in which we find them. 



Let us take, therefore, the sedum and the 

 samphire with their fleshy and succulent leaves 

 as special examples of the adaptation now in 

 view, and exhibited in many other strictly sea- 

 shore plants. In such plants the leaves possess a 

 very high power of absorbing moisture from the 

 surrounding air compared with those of other 

 plants. In this respect they are similar to the 

 cactus and other succulent plants of the tropics 



