i6o BOTANY OF THE LIVING PLANT 



requirement. Thus the mixed problem is solved by a structure that 

 serves both purposes. 



Such cases as those quoted in the last paragraphs confirm the view 

 which follows inevitably from the study of the distribution of the 

 mechanical tissues generally in plants, viz. that those methods are 

 adaptive, and have been acquired in the course of Descent in accor- 

 dance with the requirements. The structure is as a rule hereditary, 

 though the conditions to which the developing part are exposed may 

 have an influence in determining the quantity of sclerotic tissue formed 

 in the individual part. There is reason to believe that the forces 

 acting on the developing part serve as stimuli, increasing or even 

 causing the formation of the mechanical tissue. This is in accordance 

 with the well known fact that those tendrils of climbers which grasp 

 a support develop much more strongly, and are able to bear a greater 

 load than those which do not. In the large woody climbers of the 

 tropics the difference is often very marked, the fixed tendril developing 

 as a large woody structure. 



While such observations are in themselves interesting in their 

 bearing on the quantity of the mechanical tissue present, they do not 

 explain the origin of the methods of its distribution. The degree 

 of parallelism which those methods show to the methods of modern 

 engineers in using their stone, steel, and concrete, is remarkable. 

 While we admire the efficiency of the result in either case, and especi- 

 ally the economy of material made possible by such methods, it is 

 to be borne in mind that undoubtedly the priority of initiative lies 

 with the Plant. For many of the methods represented by ancient 

 types of vegetation have only been adopted within the last few decades 

 by man. Nor is there evidence that engineers ever took, as well 

 they might have done, any suggestion from the study of the engineering 

 methods of Plants. What we see in the two cases is accordingly a 

 result of parallel, or homoplastic development. Similar results have 

 been acquired independently along two quite distinct lines of evolu- 

 tion. In the one case the results have followed from human calculation 

 and experiment, in the other they are described as " adaptive." But 

 it is still an open question in what degree the structures ^een in 

 plants, and so designated, arc rausally related to the requirements 

 wliicli they so effectively meet. 



