262 PHYSIOLOGICAL DEVELOPMENT. 



and supporting peltate leaves — petioles which are therefore 

 subject to equal transverse strains on all sides — the vascular 

 bundles are arranged cylindrical^, as in axes. 



Such, then, are some of the reasons for concluding that the 

 development of the supporting tissue in plants, is caused by 

 the incident forces which this tissue has to resist. The 

 individuals in which this direct balancing of inner and outer 

 actions progresses most favourably, are those which, other 

 things equal, are most likely to prosper ; and by habitual 

 survival of the fittest, there is established a systematic and 

 constant distribution of a deposit adapted to the circumstances 

 of each type. 



§ 280. The function of circulation may now be dealt with. 

 We have to consider here by what structures this is dis- 

 charged ; and what connexion exists between the demand 

 for them and the genesis of them. 



The contrast between the rates at which a dye passes 

 through simple cellular tissue and cellular tissue of which the 

 units have been elongated, indicates one of the structural 

 changes required to facilitate circulation. If placed with its 

 cut surface in a coloured liquid, the parenchyma of a potato 

 or the medullary mass of a cabbage-stalk, will absorb the 

 liquid with extreme slowness ; but if the stalk of a fungus be 

 similarly placed, the liquid runs up it, and especially up its 

 loose central substance, very quickly. On comparing the 

 tissues which thus behave so differently, we find that whereas 

 in the one case the component cells, packed close together, 

 have deviated from their primitive sphericity only as much as 

 mutual pressure necessitates, in the other case, they are drawn 

 out into long tubules with narrow spaces among them — the 

 greatest dimensions of the tubules and the spaces being in the 

 direction which the dye takes so rapidly. That which we 

 should infer, then, from the laws of capillary action, is 

 experimentally shown : liquid moving through tissues follows 

 the lines in which the elements of the tissues are most 



