THE LEAF IN RELATION TO ITS ENVIRONMENT 



297 



country. Touch a leaf-point of Mimosa, and the small leaflets fold together r 

 and the stalk to which they are attached drops suddenly. The leaflets on 

 other branches of the compound leaf act in the same way; and finally the 

 main leaf-stalk drops suddenly. The Mimosas have received poetic treatment 

 from more than one distinguished writer. Erasmus Darwin says : 



Weak with nice sense the 



chaste Mimosa stands, 

 From each rude touch with- / 



draws her timid hands ; 

 Oft as light clouds o'erpass 



the summer glade, 

 Alarmed she trembles at the 



moving shade ; 

 And feels, alive through all 



her tender form, 

 The whispered murmurs of 



the gathering storm ; 

 Shuts her sweet eyelids to 



approaching night, 

 And hails with freshened 



charms the rising light. 



The movements of 

 so-called sensitive plants 

 are probably in part due 

 to a peculiar modifica- 

 tion of certain of their 

 leaf-cells, which in the 

 Mimosas.' at least are 

 so constructed that 

 delicate threads of pro- 

 toplasm pass through 

 their walls and maintain 

 a connection with the 

 living matter of adjoin- 

 ing cells. Thus the 

 effects of a touch on one 

 part of a leaf may be 

 transmitted all over it ; 

 and if, as is not un- 

 likely, these perforated cells are distributed through the stem and branches 

 as well as the leaves, the effects spoken of may be carried to every part 

 of the plant. 



The drooping of the leaf-stalk (which, as in the leaf of Mimosa pudica, 

 may alter its angle with the stein from ninety to thirty degrees) is caused 

 by a beautiful piece of mechanism. At the base of the leaf-stalk there is 



Photo by] [E. Step. 



FIG. 363. TEASEL (Dipsacus sylvestris). 

 Showing the protective spines on the stem, and the spiny bracts inter- 

 spersed with the flowersiin the flower-head. 



