EFFECTS OF GROWTH AS SEEN IN MOVEMENTS OF PLANT-ORGANS. 13 



In the preceding few cases, the organs grow in accordance with the 

 external stimuli ; but if the relative positions of plants with respect to 

 light, gravitation, &c, be altered, then the organ moves or bends out of 

 its original direction to try to put itself in harmony with it. 



These external stimuli, therefore, first, induce growth, and then, when 

 the direction is changed, the plant-organs respond and move themselves 

 so as to be in adaptation with it. Thus, if cabbage or other seed be grown 

 in a pot, upon which the light falls from one side, the seedlings, as they 

 come up, at once grow towards it. If the pot be reversed in position, the 

 hypocotyls begin to bend, about one-third of their length from the 

 cotyledons, the curvature extending downwards till the middle of the 

 concave side is about half-way. There is no circumnutation, but a 

 bending solely in a vertical plane. This phototropism may be repeated 

 again and again if the pot be repeatedly turned half-round. 



How are we to account for the fixed and hereditary habit of ivy and 

 Ampelopsis turning their shoots in the direction of the less illuminated 

 side? 



Comparing the climbing position of the stem with the upper free- 

 growing flowering branches of ivy, the latter have just the reverse 

 habit, for they grow outwards, away from the trunk and branches of the 

 tree to which the lower part of the stem clings. Similarly in the Virginia 

 creeper, the tendrils were originally adapted to grasping twigs and climbing 

 by that means, as do vines, to which the Ampelopsis is closely allied. 



There seems to be but one answer, viz., it is an acquired and now 

 hereditary habit. A species of Trichosanthes of the cucumber family, 

 cultivated in a frame, accidentally found its tendrils pressing against the 

 brick wall. Contrary to its normal habit, it at once began to make 

 adhesive pads ; and so we may imagine that the ancestor of the Virginia 

 creeper first acquired this habit in a similar way, the tendrils being now 

 fixed to the wall by adhesive pads ; so that its illumination is one-sided. 

 This unequal illumination we must assume caused a response to arise 

 which now induces the shoots to grow or turn towards the darker side. 



Of course, the origin of all epiphytal plants which cling to the boughs 

 or trunks of trees by means of aerial roots will receive the same interpreta- 

 tion. Thus a tropical epiphytal orchid begins by the seeds being blown 

 up to and resting on the boughs. It could not long remain there unless 

 its roots could cling to the bough. We must assume, therefore, that they 

 turned towards the bough, as the radicle of the mistletoe does, that is the 

 less illuminated side, and adhered by clasping and cementing epidermal 

 cells. 



Hydeoteopism and Helkoteopism.* 



Both moisture and gravitation play a great part in determining the 

 direction of root growth. The latter is a feeble force and easily counter- 

 acted by the former, as Sachs has so well shown f ; but when roots are 

 permeating a uniformly moist soil, as they arise endogenously at right 



* I suggest this word, as Geotropism may be partly due to darkness and moisture 

 besides gravitation. The Greek verb helko, to " attract," is used of the magnet, as 

 well as of the " drawing " down of a balance by weight, i.e., gravitation. 



f Phys., p. 715, fig. 404. 



