THE FOOD OF PLANTS. 103 



special interest to examine the sporophores of varieties of Cladonia 

 and the dorsiventral thallus of species of Sticta. 1 



1 On the structure and mode of life of lichens, see de Bary, Comparative 

 Morphology and Biology of the Fungi. 



41. Experiments with Carnivorous Plants. 



Drosera plants which it is desired to employ for physiological 

 experiments are best cultivated in flat earthen vessels on mois-t 

 Sphagnum under bell-glasses. The Droseras are frequently to be 

 found in large quantities on swampy, boggy soil ; for cultures- we- 

 need only lay a clump of the plants on the Sphagnum. It is well' 

 known that when the leaf of Drosera is stimulated, the lamina 

 curves inwards and the tentacles lay themselves together. We 

 first place a fragment of raw beef of the size of a pin's head on- 

 the middle of the leaf of a healthy specimen of Drosera rotundi- 

 folia. After some time (in my experiments at 20 C. in twenty-four 

 hours) all, or almost all, the tentacles have bent inwards. They 

 now surround the fragments of beef ; the secretion of their glandu- 

 lar heads exerts a solvent action on the proteid ; l but ultimately 

 (in my experiments at the end of forty-eight hours) the tentacles 

 straighten out again. If we place on a Drosera leaf non-nitrogen- 

 ous inorganic or organic bodies (1 experimented with fragments 

 of glass and pellets of paper), the tentacles certainly bend inwards, - 

 as before, but it is readily cfetermined that such substances induce 

 movements much more slowly than fragments of meat. It is par- 

 ticularly to be observed that whether chemical stimulus (by 

 fragments of meat) or contact stimulus (by fragments of glass) is- 

 applied to the Drosera leaf, tentacles are thrown into movement, 

 owing to transmission of stimuli, whose glandular tips have not 

 been directly brought into contact with the substance acting as an 

 irritant. 



If we examine tentacles of Drosera under the microscope (we 

 may mount them in a drop of chloral hydrate solution to render 

 the tissues transparent), we find that the stalk of the tentacle is 

 composed of longitudinally elongated cells ; the stronger stalks are 

 traversed by an axile strand of spiral vessels. The head of the 

 tentacle consists centrally of spirally thickened elements, .which- 



