66 FUNCTIONS OF EOOTS. 



Functions of Boots. 



129. Roots fix the plant, either in the soil or by attachment to 

 other bodies. They absorb nourishment by a process of imbibition 

 or endosmose (^[ 27), through their spongioles or cellular extremities. 

 The experiment of Duhamel and Senebier, conducted by inserting at one 

 time the minute fibrils alone into fluid, and at another, the axis of the 

 root alone, showed clearly that the cellular extremities were the chief 

 absorbing parts of the roots. Hence the importance, in transplanting 

 large trees, of cutting the roots some time before, in order that they 

 may form young fibrils and spongelets, which are then easily taken 

 up in an uninjured condition, ready to absorb nourishment. 



130. The elongation of the roots by their extremities, enables them 

 to accommodate themselves to the soil, and allows the spongioles to 

 extend deeply without being injured. Eoots in their lateral extension, 

 bear usually a relation to the horizontal spreading of the branches, so 

 as to fix the plant firmly, and to allow fluid nutritive substances to 

 reach the spongioles more easily. It is of importance to permit the 

 roots to extend easily in all directions. By restricting or cutting the 

 roots, the growth of the plant is to a certain degree prevented, although 

 it is sometimes made to flower and bear fruit sooner than it would 

 otherwise have done. The system of restrictive potting, formerly 

 practised in green-houses, often destroyed the natural appearance of 

 the plants. The roots filled the pots completely, and even raised the 

 plants in such a way as to make the upper part of the root appear above 

 the soil. 



131. To roots there are sometimes attached reservoirs of nourish- 

 ment, in the form of tubercules, containing starch and gum (fig. 121), 

 which are applied to the nourishment of the young plant. These are 

 seen in the Dahlia and in terrestrial Orchids. In epiphytic Orchids, 

 on the other hand, the roots are aerial, and the stems are much 

 developed, forming pseudo-bulbs. Upon the roots of Spondias tuberosa 

 there exist round black-coloured tubercules, about eight inches in 

 diameter, consisting internally of a white cellular substance, which is 

 full of water. These tubercules seem to be intended to supply water 

 to the tree during the dry season. They are often dug by travellers, 

 each of them yielding about a pint of fluid of excellent quality. 



132. Roots also give off" certain excretions, which differ in different 

 species. These are given off by -a process of exosmose (^[ 27), and 

 consist both of organic and inorganic matter. They were examined 

 by Macaire and Decandolle, and at one time they were thought to be 

 injurious to the plant, and by their accumulation to cause its deterio- 

 ration. It was also supposed, that while they were prejudicial to the 

 species of plant which yielded them, they were not so to others, and 

 that hence a rotation of crops was necessary. Daubeny and Gyde 



