268 THE BIOLOGY OF FLOWERING PLANTS 



spite of the small number of species, the ways in which the 

 insect prey is captured and utilised are very diverse. 



(a) Droseracese. — Drosera, the sundew, is a cosmo- 

 politan genus and has most numerous representatives in 

 the Cape and Australia. It is represented in Britain by 

 two species and a hybrid. The sundews are small plants 

 of wet peaty moors. Their leaves form a rosette from 

 which rise in summer the flowering stems with cymes of 

 white flowers. The root system is poorly developed. The 

 leaf is stalked and terminates, in the commoner British 

 species, in a rounded blade of a reddish hue. This blade 

 is beset with numerous " tentacles." The tentacle is an 

 emergence of pecuUar structure. The stalk is traversed by 

 a strand of tracheids which, in the club-Hke head of the 

 organ, ends in a massive group. The tracheid group of the 

 head is surrounded by three layers of cells, the outer secre- 

 tory, filled with red sap, the inner a bundle sheath. The 

 tentacles at the margin of the leaf have much longer stalks 

 than those of the central region, and the glandular head is 

 asymmetrical. The lower surface of the leaf is devoid of 

 tentacles. The head is normally covered by a glutinous 

 and sticky secretion, and this entangles small insects 

 alighting on the leaf — it is not known whether or not they 

 are definitely attracted thither. The glands which the 

 insect touches immediately begin to pour out a much more 

 abundant secretion. If marginal tentacles are aflFected they 

 begin, in less than a minute, to bend over at their bases 

 towards the centre. They bring the insect in contact with 

 the central tentacles, which, in their turn, are stimulated to 

 secrete. These tentacles do not move, but they communi- 

 cate a stimulus to the unaffected marginal tentacles which 

 now close in. In the course of a few minutes all are bent 

 over to the centre of the leaf, and their approach is intensified 

 by an incurving of the leaf margin. The insect is entirely 

 covered and submerged in the secretion. 



The movement of the tentacles is determined by two 

 different stimuli— by contact and by the chemical action of 

 nitrogenous compounds. These stimuU act either together 



