90 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 
stage, this is the proper place to mention the fact that in them positive geotropism 
appears to be completely neutralized. The growing rootlets which spring from the 
seed, and the absorptive cells produced from minute tubercles, grow upwards if 
placed on the under surface of a branch, horizontally if placed on the side, and 
downwards if on the upper surface. Thus, whatever the direction, they grow 
towards the moist bark which affords them nourishment. 
Positive geotropism seems to be quite abolished also in those marsh-plants 
which live under water. When, for instance, the seed of the Water-chestnut 
(Trapa natans) germinates under water in a pond, the main root emerges first from 
the little aperture of the nut and begins by growing upwards. Soon the smaller 
seale-like cotyledon is put forth, whilst the other, which is much larger, remains 
within the nut. The whole plant so far is standing on its head, as it were, and 
is growing upwards with its principal root directed towards the surface of the 
water. Gradually the leafy stem emerges from the bud between the two coty- 
ledons, and likewise curves upwards and grows towards the surface, whilst an 
abundance of secondary roots is developed at the same time from the main root. 
Their function is to absorb nutritive substances from the water around, now that 
the materials for growth stored in the seed are exhausted. Finding an aqueous 
solution of food-salts everywhere these roots grow in all directions, upwards, 
downwards, or horizontally to right or left, forwards or backwards, only they 
carefully avoid touching one another or interfering with each other’s sphere of 
absorption. It is not till much later that the main root changes the direction of 
its apex and bends downward. New roots are then produced from the stem; but 
this subject has no further bearing on the problems at present before us. 
The movements of roots, as they grow in earth, suggest that they are seeking 
for nutriment. The root-tip traces, as it progresses, a spiral course, and this 
revolving motion has been compared to a constant palpitation or feeling. Spots 
in the earth which are found to be unfavourable to progression are avoided with 
care. If the root sustains injury, a stimulus is immediately transmitted to the 
growing part, and the root bends away from the quarter where the wound 
was inflicted. When the exploring root-tip comes near a spot where water 
occurs with food-salts in solution, it at once turns in that direction, and, when it 
reaches the place, develops such absorptive cells as are adapted to the circum- 
stances. 
As has been mentioned before, the roots of most land-plants bear root-hairs on a 
comparatively restricted zone behind the growing point (see fig. 12°), and these 
hairs have only an ephemeral existence. As the root grows and elongates, new 
hairs arise (always at the same distance behind the tip), whilst the older ones 
collapse, turn brown, and perish. In ground which contains on every side food-salts 
in quantities adequate to the demand, and sufficient water to act as solvent and as 
medium for the transmission of the salts, the absorptive cells are rarely tubular, but 
exhibit themselves, as already described, in the form of flat cells destitute of outward 
curvature. This is the case, for instance, with those Alpine plants which grow in 
