MECHANICAL STRUCTURES IN THE VEGETATIVE ORGANS. 387 



remarks : " But the cultivated plant shows a tendency to twine. 

 It is possible that it has lost the twining habit, so common to the 

 Asclepiadaceae, through change in environment in some of the districts 

 where it has been collected."* The same remark may be made of our 

 sea-side Convolvulus [C. Soldanella), which grows in the sand, and is 

 rarely known to cHmb. Dwarf French beans are in the same condition. 



The reader will find in my book f how experiments have proved 

 that plants at once respond to artificial strains, and put on mechanical 

 supportive tissues to resist them. 



The climbing habit is easily acquired in forests and shady places, 

 as' both Fritz Muller and Prof. Warming state. The celebrated 

 surgeon J. Hunter observes if " a bean be weakly, as when grown in 

 the shade, if a stick is put into the ground close by it, it will twine 



round it in loose spiral turns." % I have proved this to be true by 

 experimenting with the periwinkle growing in almost total darkness.§ 



Leaves. — Supporting m^echanisms are to be seen in both the petioles 

 and blades of leaves. As a broad blade has to be extended at right 

 angles to incident hght, it is obvious that it must be supported, 

 and as the leverage is of the third kind, the fulcrum being the 

 place of attachment to the branch, the weight acting from the 

 centre of gravity of the blade, the power resides in the petiole. Here, 

 therefore, must be the greatest point of strength, just as it is in a 

 bough of a tree and in decHnate stamens. 



The petiole is so constructed as to secure this leverage. The 

 vascular bundles may be in a circle, just as in the stem (maple), 

 but more usually it has at least three, the largest being below, and 

 two smaller ones above, as Darwin figures them in Solanmn jas- 

 minoides in his " Climbing Plants." His figures are here reproduced 

 (figs. 99 and 100 A and B). His description is as follows The 

 flexible petiole which has clasped an object increases much in thick- 

 ness (fig. 100), and its structure is greatly changed. The ordinary state 

 of the petiole is represented by (A). In the section of the petiole (B), 

 which had clasped a stick for several weeks, the two upper groups of 

 fibres had much increased, the semilunar band below has been 



A 



Fig. 100. 



* Bot. Mag. tab. 7925. 



f The Origin of Plant Structures, p. 204 ff., 225. 

 i Memoranda of Vegetation, p. 7. 

 § Heredity of Acquired Characteys. 



