TROPMOLUM,- GOURD, ETC. 713 



subsequently become curved convex outwards, the apex of the shoot thus turning 

 away from the light, while the leaf-stalks, as in the Ivy and in all similar cases, 

 are decidedly positively heliotropic, and the laminae of the leaves are placed at 

 right angles to the incident light. The apices of the shoots turned away from 

 the light never become quite horizontal, however, probably because their geotro- 

 pism works to the contrary; but in the open, where the light is more intense, 

 the negative heliotropism overcomes the geotropism, and the shoot-axes lie hori- 

 zontal and close to the earth. It is in consequence of this .effect of light also 

 that the plant is -able to apply itself closely to the vertical surface of a wall, and if 

 provided with a trellis in order to hold it fast, to climb up it. If, on the other 

 hand, the plant developes in the shade, where the light is feeble, the shoots grow 

 erect, and in fact become even positively heliotropic, and it is further to be mentioned 

 that even those shoots which are curved towards the shaded side when the illumin- 

 ation is strong on one side possess no permanent dorsi-ventrality. If the plant is 

 turned round so that what was hitherto the shaded side is now strongly illuminated, 

 the part of the stem already fully developed maintains its curvature, it is true, but the 

 younger internodes which are still growing become curved backwards, with the 

 apex of the shoot turned away from the window. With TropcBolum, however, such 

 experiments must be made in bright summer weather ; in the autumn, when the light is 

 feebler, the shoot-axes are always positively heliotropic. 



The Gourd plant also behaves in all essential points like the Indian Cress. 

 That its stem, at tirst orthotropic, subsequently lays itself horizontally an.d prone on the 

 earth is an effect due to strong light : plants cultivated in the shadow of a room remain 

 erect for a long time, or curve but slightly obliquely away from the window. For 

 these reasons Gourd plants are able to climb up vertical walls provided with a 

 trellis, the apices of the shoot becoming so to speak pressed to the wall by the light, 

 and are then able to fasten themselves to the lattice by means of their tendrils. In 

 the open, where the light is strong, the long shoot-axes of the Gourd lie horizontally 

 on the earth; but if the plant grows in the shade of undergrowth, the apex 

 erects itself and, with the aid of the tendrils, climbs between the branches of the 

 underwood into the full light. In the case of Gourd-shoots lying horizontally on the 

 earth, the apical portion bearing the terminal bud is erected in ordinary daylight so 

 as to assume approximately the form of a horse's neck, the bud, bent sharply down- 

 wards, representing the head. It is now easy to observe that on very bright days 

 this horse's neck lays itself flatter on the earth, while in dull weather it is more 

 erect. If the apex of the shoot is directed through a hole into the dark cavity of a 

 box, as in Fig. 238 (p. 354), it then goes on growing perfectly upright. 



It would be very easy to quote numerous other instances of plants which behave 

 towards different intensities of light like the Indian Cress and the Gourd : in addition 

 to the common Vine, I will only name Glechoma hederacea, one of the Labiatse. In 

 connection with the phenomena just mentioned, induced by light in highly organised 

 plants, however, I must return once more to Marchantia, to add that the described 

 plagiotropic position of the flat shoots in this plant also is assumed only in a 

 sufficiently strong light; if the light is unilateral and feeble, the Marchantia 

 shoots erect themselves, but without losing their typical broad form. When the 

 light is very feeble, thie shoots at length remain narrow and become nearly 



