GERANIUMS OR CRANESBILLS 217 



even when the " stigma was abundantly smeared with its own 

 pollen." It is difficult to understand how such an extra- 

 ordinary characteristic as this is developed, but it is not 

 uncommon, particularly amongst the Orchidaceae. 0. corni- 

 culata is endemic. 



The genus Oxalis shows well the folding of the leaves known 

 as the " sleep of plants." The head of the petiole droops, 

 and each blade is folded along the midrib, so that only the 

 edges, and not the surfaces of the leaf are exposed to the sky. 

 The position is, no doubt, as Darwin suggested, a method of 

 protection against excessive loss of heat by radiation to a clear, 

 cold sky ; but when this has been said, many things still 

 remain to be explained. The movement is generally stimulated 

 by the oncoming of darkness. However, it will take place at 

 regular intervals, for a time at least, in prolonged darkness, or 

 in constant illumination. There is, therefore, a tendency in 

 the plant to carry on the regular changes of position, in the 

 absence of the stimulus ; but sooner or later unnatural con- 

 ditions produce disease, or an abnormal response in the leaves. 

 The mechanism, by means of which the movements are carried 

 on, is fairly well known. In many plants, it consists of a 

 cushion of cells on the petiole, which can be distended or 

 contracted by the injection of fluid into them, or its removal 

 from them. When the cushion is in a state of turgidity, 

 the leaf is raised ; when it is flaccid, the leaf droops. The 

 " sense-organs " if so they may be termed by means of 

 which the plant can distinguish light from darkness, or 

 variations in light, are as yet very imperfectly known. 

 Haberlandt, however, has endeavoured to show that there are, 

 in many leaves, transparent microscopic lenses which he terms 

 " ocelli " (little eyes), whose function is to detect the alteration 

 in the amount of light received by the leaf, and thus, perhaps, 

 receive a stimulus, which will automatically control the motion 

 of the leaf. 



This " Somnus plantarum" as Linnaeus termed it, may be 

 readily witnessed in 0. corniculata. 



