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 

 scale-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 3 ), 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 i 



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