670 JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Gravity acts as a stimulus, under which the plant develops these 

 supportive tissues, in order to resist the pull earthwards, which is always 

 at work. 



There is ample evidence to prove that the protoplasm of plants 

 resembles that of animals in responding to the influence of external forces, 

 and strives to acquire and to sustain an equilibrium with them. The above- 

 named tissues are the result of this ''efitbrt," though they, of course, can- 

 not do this suddenly, like a man using his muscles to prevent himself 

 from faUing if he have lost his balance ; but the result is no less effective, 

 though it be executed by the slow method of growth ; just, indeed, as an 

 athlete's muscles will gradually enlarge under repeated exercise. 



On the contrary, in submerged water-plants, this effort is not required ; 

 and consequently the supportive tissues almost entirely fail to be formed ; 

 for such plants (with the aid of air in their lacunae) are of much the same 

 specific gravity as water, and therefore miss the external stimulus of 

 any strain to which they can respond. 



Water-plants, therefore, are always more or less degraded in structure, 

 just as muscles become reduced in size or atrophied if not employed. 



Similarly, with large and massive cellular plants, as of a Cactus, little 

 or no strengthening material is required, such as woody tissue, conse- 

 quently the fibro -vascular bundles are, in certain ways, of a degraded 

 character. When, however, even such stems are artificially subjected to 

 strains, they at once begin to develop wood ; as Mr. Herbert Spencer found 

 to be the case in the experiments carried out by Mr. Croucher of Kew : — 

 " In such types as Cereus and Opuntia we see as in the [fleshy] Euphorbias, 

 that where little stress falls on the vessels, little deposit [of wood] takes 

 place around them ; while there is a deposit where there is much stress."* 



Analogous results take place under degeneration brought about by 

 other causes than water. Thus M. Costantin's experiments proved that 

 when normally aerial stems are grown underground the supportive 

 tissues become at once arrested, and the new portions begin to approach 

 in structure to rhizomes, which are normally subterranean and develop 

 little wood. " Use " and " Disease " are, therefore, quite as applicable to 

 plants as to animals ; for while the muscles in the arm of a blacksmith 



* On Circulation and the Formaticm of Wood in Plants, Trans. Linn. Soc. 1866, 

 p. 405. . 



>Per. 



Fig. 274. — Trans, sect, of stem of Dead- 

 nettle, showing the distribution of 

 collenchyma-strands at the corners 

 and sides, together with 8 fibro- 

 vascular bundles within them. 

 (After Kerner and Oliver.) 



Fig. 275.— Trans, sect, of flower-stalk 

 of Ixia. Ep., epidermis ; Chi., cells 

 with chlorophyll; Pgr., pericycular 

 sheath of elongated sclerenchyma ; 

 F. V. b., fibro - vascular bundles ; 

 Grd. tis., ground tissue. 



