6S TISSUES 



pressure within the latter rises to such a height that the inter- 

 cellular spaces are in danger of becoming injected. The insectivorous 

 leaf of Din mini muscvpula is provided with a special motor-tissue 

 located in the midrib of the distal half of the lamina which is 

 responsible for the characteristic snapping together of the two halves 

 of the trap: the motor-cells can, however, only lie set in action 

 through the mechanical stimulation by shock or contact of one 

 of the six sensitive bristles situated on the upper surface of the leaf. 

 In these cases the direct control exerted by one organ upon some 

 physiological function of another takes the form of an act of stimula- 

 tion. The special condition of the controlling organ which results in 

 the stimulation of the executive structure may itself be induced by an 

 external stimulus : witness the case of Dionaca. In all such instances of 

 direct correlation between separate organs, the requisite stimulus must 

 be transmitted through the intervening tissues ; transmission of stimuli 

 may take place in a variety of ways. 



Correlation of tissues does not always involve either the adaptive 

 regulation of one function by another or the co-operation of distinct 

 functions. Quite apart from pathological conditions, it is certain that 

 in a perfectly normal plant the various tissues inevitably compete with 

 one another to a certain extent ; usually it is a question of competition 

 for space or for a supply of plastic materials. The success of a 

 particular tissue is naturally dependent upon a variety of circum- 

 stances. In stems and other inflexible organs a certain amount of 

 competition not infrequently takes place between the mechanical and 

 the photosynthetic tissues : for the general structure of both these 

 systems necessitates their being situated as near the surface as possible. 

 Schwendener, however, has shown that in the haulms of Grasses the 

 issue of the struggle between the systems is determined by an 

 external factor, namely, illumination. In the exposed portions of the 

 haulm that is to say, in the region in which photosynthesis is favoured 

 by strong illumination the photosynthetic tissue gains the upper 

 hand and occupies the periphery to the entire exclusion of the 

 mechanical cells. In those parts of the haulm, on the other hand, 

 which are enclosed by the sheathing leaf-bases, the photosynthetic 

 system is placed at a disadvantage, owing to the relatively feeble 

 illumination, and accordingly gives way to the mechanical tissue. 



In conclusion the influence of external conditions in general upon 

 the physiology and morphology of vegetable tissues must be briefly 

 considered. Every tissue displays, in its hereditary morphological 

 characteristics and physiological properties, a very evident adaptation. 

 both to the general conditions of existence common to all plants, and 

 to the special features of the edaphic and climatic environment peculiar 



