THE LIFE -HISTORY OF PLANTS. 



371 



showing that neither light nor moisture of itself 

 deflects roots from their course. In Fig. 52 is also 

 shown the gradual uplifting of the stem and the 

 speading of the leaves towards the light by " helio- 

 tropism." In Fig. 53 the stem is at first "procum- 

 bent," the attraction of gra^dtation is more powerful 

 than the upward tendency of growth ; but the latter 

 gets the upper hand and the stem turns upwards. 



One - sided Growth, or 



that in which the energy of 

 growth is much larger on the 

 one side than. on. the other is, as 

 has just been remarked, due to 

 the unequal pressure of some 

 obstacle which checks growth 

 on the one side, which conse- 

 quently remains flat or concave, 

 while on the other side, where 

 growth is not checked, it be- 

 comes convex. This is the ex- 

 planation of the coils and twists 

 we often see, and in which the 

 side that has been pressed upon 

 or obstructed in its growth 

 acts like a curb upon the other 

 side, where growth is going on 

 more freely, and causes it to 

 assume a convex form (Fig. 54). 

 The upward turn of the stem in 

 Fig. 53 is accounted for by the 

 more rapid growth of the lower 

 surface of the stem than of the 

 uppter, which is exposed to the 

 light and consequently re- 

 stricted in its development. 



Heliotropism. — Helio- 

 tropic " curvatures, or such as 

 are due to the tendency of a 

 growing plant to turn its leaves 

 to the direction whence the light 

 comes, also cause one-sided growth. Here the ne- 

 cessity for light and the consequent storage of food 

 induces a bending of the side exposed to the hght, 

 although when all conditions are equal the actual 

 amount of growth is most active on the side turned 

 from the light. Most plants exhibit these so-called 

 heliotropic curvatures to a marked degree, and not 

 only are the stems or branches curved in accordance 

 with the direction whence the light falls, but the 

 leaves have the power of so altering their position as 

 to place themselves in the most favourable condition 

 as respects the light. This is especially observable 

 in the leaves of Silver Firs, the grey under surfaces 

 of which are much more conspicuous during bright 



Fig. 54.— Stem of a Climbiug- Plant, to show 

 the manner in which it ascends its support. 



light than at duller periods, owing to the pov. or the 

 leaves have of twisting themselves round, and of 

 raising or depressing themselves as the case may be. 

 The leaves of a species of Pothos, in which the broad 

 stalk 'is jointed transversely to the blade, also show 

 Ihese heliotropic movements very markedly, the leaf 

 assuming almost every possible angle with reference 

 to its supporting stalk, in accordance with the dif- 

 ferent directions from which 

 the light falls. 



Tlie Ivy affords a curious 

 illusti'ation of what has been 

 somewhat absurdly called 

 "negative heliotropism." In 

 it the shoots when growing 

 against a wall bend away from 

 the light when they get the 

 chance, probably for the purpose 

 of securing a firmer hold on the 

 rock or wall. 



Alternations of Growth. 



— Intermissions of growth such^ 

 as those alluded to in the last 

 paragraph have reference more 

 particularly to time, but there 

 are other alternations of growth 

 connected with particular parts 

 of the plant. Gi-owth in length, 

 as has been pointed out, occurs 

 principally at certain well- 

 defined growing points, as ad- 

 jacent to the tip of the root, at 

 the points of the buds, and so 

 forth. Besides this, there is 

 what is called "intercalary 

 growth," in which centres of 

 active gi'owth are insinuated in 

 the midst of tissues otherwise 

 passive. The occasional and sud- 

 den outgrowths of buds or other 

 excrescences from the surface of 

 leaves or other parts where such growth is unusual 

 may be explained in this manner ; but such growth 

 is more or less irregular, and not to be confounded 

 with the alternation of grow^th— now in this part, now 

 in that — which constitutes so marked a feature in 

 the regular course of things. In a rapidly- growing 

 shoot, for instance, it is found by direct observation 

 and measurements that the greatest activity of 

 growth at one time is along one particular line or 

 strip of tissue (the others being comparatively or 

 completely at rest), at another time along another, 

 and so on round the shoot, until the whole circuit 

 has be(3n accomplished. Such alternations in the 

 direction and locality of active growth appear to be 



