DROSERA ROTUNDIFOLIA. 



339 



The structure and office of the prehensile and digestive appara- 

 tus are now to be illustrated by the following examples : 



889. Drosera rotundifolia, or round-leaved sundew, grows 

 abundant!}' in northern peat-bogs and in sand mixed with vege- 

 table mould, both in the Old World and the New. The plant 

 has a few (4 to 12) leaves, arranged in a flat tuft at the base of 

 the flower-stalk, and narrowed at their bases into hairy petioles. 

 The most striking character of the leaves is the thick clothing 

 of peculiar hairs, otherwise known as tentacles or glands, from 

 the tip of each of which exudes a drop of a clear viscid 

 liquid. These hairs are complicated 



in structure. They contain all the 

 histological elements proper to the 

 leaf itself; for this reason it has been 

 thought by some that they should be 

 regarded as processes from the leaf 

 rather than as hairs. The marginal 

 tentacles are long, have purple stalks, 

 and are terminated by elongated pur- 

 ple glands ; those towards the middle 

 of the leaf are shorter, have greenish 

 stalks and ovoid glands. Each gland 

 consists of a double layer of polygo- 

 nal cells which surround a central 

 bod} r composed of elongated cells 

 and a few tracheids. The proto- 

 plasmic lining of all the cells is transparent and thin, and the 

 cavity is filled with an homogeneous purple fluid. The tra- 

 cheids pass by insensible gradations into minute spiral ducts. 



890. The mode of action in Drosera is as follows : When 

 a small object is placed on the middle glands, a sluggish move- 

 ment is soon detected in the marginal tentacles. If the object 

 is a fragment of animal matter, the motor impulse is commu- 

 nicated rapidly, and the marginal tentacles curve sharply over 

 upon the fragment, bringing the glands in contact with it. The 

 blade of the leaf also sometimes becomes curved, forming a shal- 

 low cup. Inorganic and such organic matters as are not acted 

 on by the secretion from the glands act more slowly in causing 

 movement of the tentacles than do soluble organic substances, 

 and no movement follows unless the object rests upon the 

 glands themselves, not merely on the secretion which covers 



FIG. 153. Drose-a rotnndifolia. View of leaf from above. (Darwin.) 



