128 



HUTCHINSON'S POPULAR BOTANY 



FIG. 162. RHODO- 

 DENDRON. 



Part of section through leaf, 

 showing a single stoma with 

 its guard-cells. Magnified. 



to develop themselves in either of the two sides ; 

 the stomata * being always formed on the upper 

 surface, under the influence of light, and the . . . 

 [rhizoids] proceeding from the lower towards darkness. 

 But if the surfaces be reversed after the reproductive 

 organs have been developed to a certain point, so that 

 the stomata be directed towards the ground, and the 

 . . . [rhizoids] be made to rise into the air, the little 

 plant will right itself, by twisting itself round, so as 

 to bring its surfaces to their former position. Further, 

 when plants of a higher description are grown in dark- 

 ness, the stomata are developed very imperfectly, or 

 not at all. Thus we have an example of the very im- 

 portant effects of the stimulus of light upon the veget- 

 able structure, not only in covering its actions, but in 

 influencing its development" (Vegetable Physiology and Botany). 



"We have said that the stomata are the organs through which the plant 

 transpires. The condensation of water on the glass surface of an ordinary 

 fern-case is a familiar instance of transpiration ; though doubtless some of 

 the vapour is due to evaporation from the soil. By placing a piece of 

 cardboard, through which a small hole has been made for the insertion of a 

 well-developed leaf-shoot, over the mouth of a tumbler of water, and covering 

 the whole (leaf-shoot and tumbler) with a bell-glass, evaporation will be 

 prevented, and the watery deposit forming on the inside of the glass will 

 soon furnish proof that water is transpired from the leaves. Hence the 

 necessity for keeping the roots of plants well supplied with water ; for if the 

 loss by transpiration be greater than the quantity supplied by the roots, the 

 conducting parts (as the stem and branches) quickly suffer ; and when at 

 length the evaporation from the more delicate organs can no longer be 

 compensated, they lose their stiffness or turgidity, hang down from their 

 own weight, and wither. The flagging of leaves, so 

 often noticed in the potting and bedding-out of plants, 

 is due to the same cause. The delicate root-hairs, by 

 which alone absorption of the soil is effected, get de- 

 stroyed in the process of transplanting, and thus the 

 upward flow of crude sap to the leaves is temporarily 

 arrested. In cases of this kind, transpiration should be 

 artificially checked by shading the plants from the light 

 till such time as new root-hairs have been found, when 

 absorption will again take place. 



FIG. 163. FIELD 

 HORSE-TAIL. 



Section through part of stem 

 of Equisetum arvense, show- 

 ing a stoma and its connec- 

 tions. Magnified. 



* Or, rather, stomata-jtores. They are really pores in the outer- 

 most layer of tissues (for the thallus has no true epidermis), and each 

 pore leads into an air chamber much larger than itself, in which is 

 contained the assimilating tissue of the thallus. 



