MECHANICAL STRUCTURES IN THE VEGETATIVE ORGANS. 381 



MECHANICAL STRUCTURES IN THE VEGETATIVE ORGANS 

 OF PLANTS, DEVELOPED IN RESPONSE TO PHYSICAL 

 FORCES, COMPARED WITH SIMILAR ONES EMPLOYED 

 IN ENGINEERING. 



By Rev. Prof. G. Henslow, M.A., F.L.S., V.M.H. 



[Read October 20, 1914 ; Dr. F. Keeble, F.R.S., in the Chair.] 



Animal mechanics have long been observed and described, but 

 vegetable mechanisms have been only incidentally alluded to in 

 botanical works.* 



I propose now giving examples of certain mechanical structures 

 in plants which can in several cases be illustrated by like constructions 

 required in engineering, when similar strains have to be met : as in 

 building bridges, in the erection of iron pillars to support heavy 

 roofs, as well as in the construction of levers, boilers, &c. 



Purposeful mechanics are to be seen in all organs of plants, viz. 

 roots, stems, and branches ; petioles and blades of leaves ; floral 

 organs, fruits and seeds. 



Resistance to Gravity by Stems. — Any plant which grows above 

 the ground at once " feels " the everlasting " pull " of gravity, and 

 unless the stem can support its own weight, 

 down it must go. Purely cellular tissue, if 

 turgid, may be enough, as in the stems of 

 mushrooms ; but in flowering plants this is 

 generally much strengthened by a number 

 of woody, vascular strands, arranged in one 

 or more circles, especially near to the cir- 

 cumference, as seen in a transverse section 

 of most Dicotyledons. Keener f would 

 compare the isolated bundles of an herba- 

 ceous stem to girders, but the objection to 

 this idea is that there is no stiffening tissue 

 corresponding to the iron "web" or cross- 

 bar of a girder, connecting the opposite strands regarded as the 

 "flanges." The pith offers a very feeble resistance to bending 



* I dealt with some few of the mechanisms of flowers in The Origin of Floral 

 Structures (1888), illustrating Lamium, p. 126 ; declinate stamens, pp. 81, 82, 

 126 ; and the fruit of the pear, p. 124. A chapter (xiii. p. 123) also deals with 

 the effects of strains on structures. Others will be found in the Journal 

 R.H.S. 1901. Experiments will also be found described under the heading 

 "The Resistance of Plants to Mechanical Strains" in my Origin of Plant 

 Structures, p. 203. 



t Natural History of Plants, p. 724, under the heading " Resistance of 

 Foliage-stem3 to Strains, Pressure, and Bending." 



Fig. 98. — Arrangement 

 OF Vascular Bundles 

 IN A Dicotyledonous 

 Stem. . 



