THE MOLECULAR FORCES IN PLANTS. 171 



width. We can easily prove that the slit of a stoma under certain 

 -conditions is more or less widely open, while under other condi- 

 tions it is closed. These remarkable changes are due to variations 

 in the t algidity of the guard cells and of the adjoining epidermal 

 cells of the stomatal apparatus, and, neglecting complicated de- 

 tails, we will make some experiments demonstrating the main 

 facts. 2 



We examine first a tangential section of a leaf of Amaryllis 

 formosissima, cut off in the daytime, and find the stomataopen. If, 

 on the other hand, we use a half-withered leaf of the same plant, 

 we observe that the stomata are closed. It is well to examine the 

 section dry, and only add water after we have satisfied ourselves 

 that the stomata are shut. Addition of water now causes the 

 stomata in the course of a few minutes to open widely, owing to 

 considerable increase in the turgidity of the guard cells. If, how- 

 ever, the sections are left in water for some time, the stomata close 

 again, because the turgidity of the epidermal cells also gradually 

 increases largely, so that the pressure of the guard cells is little 

 by little overcome. 



If, however, sections are prepared from Amaryllis leaves which 

 have been well sunned, it is impossible to cause the stomata to 

 close by treatment with water, since the guard cells, in consequence 

 of assimilation, contain such large quantities of osmotically active 

 substances, that they turgesce energetically enough to hold the 

 turgidity of the neighbouring epidermal cells in equilibrium. 



The stomata of other plants behave like those of Amaryllis. 

 On the other hand, we observe a difference of behaviour in plants 

 in which the epidermal cells adjoining the guard cells are of no 

 great importance in connection with the stomatal apparatus. If 

 strips of epidermis from Orchis leaves (I used Orchis mascula) are 

 examined first of all in a drop of water, it is seen that the stomata 

 never close, but remain open. If, on the other hand, the section 

 be laid on the slide in a drop of cane-sugar solution, the stomata 

 close fairly quickly. The stomata of Lilium candidum behave in 

 a similar manner. Copious supply of water to the plant, to leaves 

 removed from the plant, or to the sections, opens the stomata. 

 Withering of the leaves, or withdrawal of water from the sections, 

 reduces the turgidity of the guard cells, and causes the stomata 

 to close. 



The stomatal apparatus of many plants reacts also to changes 

 in illumination. The stomata of Amaryllis formosissima, for 



