530 SUMMARY OF CURRENT RESEARCHES RELATING TO 



roots, and has come to the conclusion that his statement of these 

 properties cannot in all respects be substantiated. He states that the 

 curvatures manifested by the roots when a foreign body is applied 

 to them on one side, and which Darwin attributes to the sensibility 

 of the roots, are really due to injury suffered by the root from the 

 cutting off of free access of air, all the tissues being found to be 

 destroyed up to the cylinder of plerome. That the cause of the 

 curvature is injury to the root-sap is shown by parallel experiments 

 in which the injury is inflicted in other ways. 



The statement of Darwin that it is only the apex of the root that 

 is susceptible is also contested. Experiments with roots from 

 which the tip had been entirely removed, showed the same geotropic 

 phenomena as others in which the roots were entire. 



The author further disputes Darwin's assertion that the apices 

 alone of roots are susceptible to change in the degree of moisture of 

 the environment ; he states, on the contrary, that the whole of the 

 growing part of the root, and not merely the tip, is affected by an 

 unequal degree of moisture in the surrounding air, curving in the 

 direction in which the air is most moist. 



Aerial Cultivation of Aquatic Plants.* — E. Mer records the 

 results of a series of experiments for the purpose of determining 

 the effects produced by growing in the air plants which ordinarily 

 grow entirely submerged in water. They were placed in a vessel 

 of water in such a way that the buds were above the surface of the 

 water, and the whole covered with a bell-glass ; others being, at the 

 same time, grown under similar vessels entirely in water. The sun, 

 in July, was powerful during the whole of the experiments. 



In Potarnogeton natans and rufescens the shoots grown in the air 

 were distinguished from the normal ones by the shortness of their 

 internodes, the smallness of the leaves, which partially remained 

 rudimentary, and the presence of numerous stomata, which were also 

 found, but in small numbers, in the newly formed branches of the 

 submerged specimens. The formation of stomata is ascribed by 

 the author to the retardation of growth and to heredity. The 

 accumulation of reserve-materials in the tissue resulting from the 

 retardation may bring about divisions in the epidermal cells, and 

 hence lead to the formation of stomata. The formation of stomata 

 only in the parts exposed to air he attributes to more vigorous trans- 

 piration. Similar causes lead to the formation of stomata on the 

 perianth-leaves of P. rufescens and the foliage-leaves of Littorella 

 lacustris. Hereditary tendency causes the localization of the stomata 

 on the upper side of the leaves in P. natans, as is usual in floating 

 leaves ; it also produces the effect in Littorella that, when grown in 

 the air, the newly formed leaves possess a larger or smaller number 

 of stomata, according as the plants grew originally at a greater or 

 less depth below the surface of the water. 



In Hydrocharis morsus ranai the size of the leaves was greatly 

 diminished, as well as the length of the leaf-stalk, the intercellular 



* Comptes Kendu9, xciv. (1882) pp. 175-8. 



