THE BUILDING-STONES OF THE ORGANIC WORLD 67 



lock in rat-trap fashion with those of the opposite side. The 

 centre of the leaf bears numerous digestive glands, and there are 

 on each half of the blade three sensitive hairs which rise obliquely, 

 but bend flat on a basal joint when the leaf closes. The blade 

 shuts up in eight to ten seconds when one of the sensitive hairs 

 is stimulated, and if an insect is' caught in the fly-trap a profuse 

 secretion is exuded from the glands ; after a week or a fortnight 

 the insect is digested and the leaf then reopens.' (Thomson.) 



It has long been known that the leaves can carry out move- 

 ments calculated to place themselves in the most favourable 

 position to the light. Haberlandt's investigations showed that 

 these movements are made by organs situated in the upper 

 epidermis, which not only perceive the light-stimulus but also the 

 direction of the light. In the simplest forms the entire upper 

 epidermis of the leaf has become a light-sensitive epithelium. 

 In other plants, for instance Fittonia verschaffelti, a few speci- 

 fically formed cells have assumed the role of primitive eyes, 

 called ocelli, like the analogous organs in the lower animals. 

 In Fittonia these ocelli consist of two cells, one above the other. 

 Protruding from the other homogeneous epidermis-cells are in 

 different places large cells with strongly vaulted outer walls, each 

 of which carries on its head a small biconvex lens-cell. The 

 interior of this lens-cell is filled with a clear, highly refractive 

 substance. Probably these ocelli were evolved from cilia. But 

 not only light-sensitive organs but also specific forms for the 

 reception of mechanical stimuli and stimuli of gravitation have 

 been made known as the result of laborious research, and thus 

 what once appeared to be a fundamental distinction between 

 plant and animal has become a new connecting link. 



Many experiments have further demonstrated the inaccuracy 

 of the assertion that only plants are able to form cellulose, for 

 this substance is not only found in the protective covering of 

 many of the lower animals but forms also the chief constituent 

 of the mantle of the tunicates. 



More vague still does the line of demarcation become when 

 we come to unicellular organisms ; for here in the majority of 

 cases it becomes really a matter of choice whether we count 

 certain organisms among animals or plants. With each advance 

 made by science it becomes clearer that a natural division of 



