Physiologie. 295 



The Stoma opens by: 



1. A change in the shape of the guard-cells, rather than 

 by stretching of their vvalls: the change is due to 



a) An increase in the depth of the guard-cells, in which 

 there is chiefly concerned 



the entire wall (except the dorsal) . . Medeola, 



the inner half Mnium, 



the outer half Funana, 



the dorsal half Lycopodium, 



the ends Osmunda. 



b) An increase in the width of the slender 



stoma Sagittaria. 



2. Stretching of the thin dorsal wall . . . Amaryllis. 



3. A change in shape, with or without much 

 stretching, at the ends of the guard-cells 

 which forces the dorsal wall, with the 

 passive middle part of the cell 



directly backward The Gramineae, 



upward and backward .... The Coniferae. 



4. Combinations of the types of 



Amaryllis and Mnium Helleborus, 



Amaryllis and the Gramineae .... Achillea. 



To some extent the different types of stomatal mechanism 

 indicate oecological adaptation: thus spring plants have gene- 

 rally stomata of the Amaryllis or Medeola type; whilst summer 

 plants, and trees at all times, have stomata of the Helleborus 

 or Achillea type, as a protection against drought. Vines. 



Darwin, F., On a Method of Investigating the Gravi- 

 tational Sensitiveness of the Root-tip. (Journ. 

 Linn. Soc. Botany. Vol. XXXV. No. 245. July 1902. 

 p. 265—274.) 



The object of this research is to confirm the results of 

 Pfeffer and of Czapek as to the geotropic sensitiveness of 

 the root-tip. The method consists essentially in fixing the root- 

 tip by means of a tube in a horizontal position whilst the 

 hypocotyl and cotyledons are free to move. It is obviously 

 necessary that the cotyledons should be so supported that their 

 weight does not interfere with the result. This object is attained 

 by fixing the seedling (Bean or Pea), by means of a pin 

 passed through the cotyledons, to the end of a counterbalanced 

 lever which can rotate both vertically and horizontally. The 

 result is that when the root begins to curve geotropically, the 

 cotyledons are moved down wards. In most cases the curva- 

 ture reached the vertical: but in several it extended conside- 

 rably beyond it, so that the cotyledons were directed upwards, 

 having travelled through more than 180° in some cases. 



Vines. 



