126 



HUTCHINSON'S POPULAR BOTANY 



floating leaves the very place where they are most plentiful in land plants. 

 Plants of the Cactus tribe (Cacteae) and some tropical Euphorbias whose 

 leaves, like those of the Cacti, hereafter to be spoken of, have been meta- 

 morphosed into spines and thorns for protective reasons develop their 

 stomata on the fleshy succulent stems. From roots if we except the 

 green-celled aerial roots of a few epiphytes, such as the Tree-orchids of the 

 tropics they are entirely absent. In the interesting Polar-plant (Silphium 

 laciniatum) the stomata are about equally distributed on both sides of the 

 broad flat leaves a very necessary provision, because of the peculiar position 

 of the leaves, both faces of which are in every case equally illuminated by 

 the sun. This is the case with most, if not all, plants with vertical leaves. 



A curious fact, not unconnected with 

 our present subject, has been brought 

 to light by Dr. M. C. Cooke, in relation 

 to Bomarea carderi, a handsome climbing 

 plant of Colombia. The plant has long 

 lance-shaped leaves, and Dr. Cooke has 

 pointed out that the under surfaces of the 

 blades of these leaves are exposed to the 

 light, owing to a twist in the leaf-stalk 

 (fig. 167). To give additional interest to 

 the discovery, a competent physiologist, 

 Mr. W. S. Gilburt, to whom specimens 

 were submitted, ascertained that the entire 

 structure of the leaves is reversed, in 

 order to fulfil the conditions of their re- 

 versed position ; the under surface being 

 smooth, and presenting the usual character- 

 istic epidermal cells of an upper surface ; 

 while the true upper surface is fitted to 

 do duty for the former. No satisfactory 

 reason has yet been assigned for the twist- 

 ing of the leaf-stalk, and if ever the phenomenon is accounted for it 

 will probably be by one who has studied the plant closely in its native 

 habitat.* 



It has -been shown that the presence of light is most essential to the 

 development of perfect and vigorously acting stomata. This fact with 

 other related facts has been well illustrated in the case of one of the 

 commonest of the Liverworts, Marchantia polymorpha (fig. 169). The young 

 plant, when first separated as a kind of bud from its parent, exhibits no 

 stomata or roots.f "It has been ascertained by repeated experiments," says 

 Dr. Carpenter, "that stomata and roots [really 'rhizoids'J may be caused 



* Freaks and Marvels of Plant Life, pp. 196, 197. 



t More correctly rhizoids. Rhizoids corresponds to the root-hairs of Flowering Plants. 



FIG. 160. STOMA OF SCOTS PINE 



(Pinus sylvestris). 



The breathing pores of plants are called stomata 

 or mouths. In this section of a leaf of Scots 

 Pine, the open stoma on the surface is seen to 

 connect with spaces between the cells of the leaf- 

 tissue. Magnified. 



