THE MEANING OF ANISOTROPY. 



699 



■which any part of a plant grows is determined by its geotropism and heliotropism, 

 and we shall see further on that other external influences also are effective to the 

 same end. ' 



Before attempting to understand this phenomenon, which prevails throughout the 

 vegetable kingdom, however, it will be well to make the fact itself clear by means of a 

 few examples. In very many land-plants the original plumule grows vertically upwards, 

 and the primary root vertically down- 

 wards, and, as we have seen, both in 

 consequence of their sensitiveness to the 

 influence of gravitation, so long as one- 

 sided illumination of any kind does not 

 deflect the vertical directions of growth 

 into oblique ones by means of heliotropic 

 curvatures. But the lateral roots of such 

 plants — e.g. Sunflower, Ricinus, Fir, &c.— r- 

 either grow horizontally or descend oblique- 

 ly, and the lateral roots of the second 

 and higher orders which arise from these 

 may grow out in all directions, according 

 to their origin. The case is very similar 



with the lateral off-shoots of the vertical 

 shoot-axis which is developed from the 

 plumule: its leaves assume an oblique or 

 horizontal position, the upper side being 

 always turned towards the light, while 

 the lateral shoots grow horizontally, or up- 

 wards at an oblique angle. 



In other cases, on the contrary, the 

 primary shoot developed from the plumule 

 at once grows in a horizontal direction, 

 and forms on its under side roots which 

 descend vertically, and on its flanks or its 

 upper side leaves, which bear horizontally 

 extended laminae on vertically erect leaf- 

 Stalks, as for example Fig. 392, and the 

 same is true, as has already been stated, 

 of the common Bracken Fern, Pteris 

 aquilina, (Fig. 55, p. 60). 



In many tuberous and bulbous plants, and particularly in the case of 

 some Aroids, such as Sauromaium, the strong leaf-stalks spring from the subter- 

 ranean central body, and likewise become raised vertically upwards, like the stem 

 of a Fir. 



It is otherwise, again, in the case of the Ivy, for instance, the shoots of which, 

 pressed closely to a wall, rock, tree-trunk, &c., grow vertically, obliquely, or hori- 

 zontally, while the petioles are turned away from this substratum in order to 

 present their laminse at right angles to the incident light; whereas the aerial 



Fig, 392. — Marsilia satvatrix, anterior portion of stem with 

 leaves (4 natural size). IC terminal bud ; bb leaves ; ff sporo- 

 carps, arising from the petioles at x. 



