26 



FUNCTIONS OF PLANTS, CONSIDERED 



into sugar, and absorbed by the embryo, the radicle being protruded at 

 one end of the grain or seed, and the plumule or commencement of the stem 

 elongated at the other. 



103. Growth in plants is eflfected, not as in animals by the expansion of 

 all the parts of the embryo, but by additions to it. Thus roots and stems 

 lengthen by matter added to their extremities ; and are thickened by layers 

 of matter deposited on their surface in the case of exogenous plants, and in 

 the interior of their stems and roots in the case of endogens. In the embryo, 

 the root first begins to move by the extension of all its parts, but imme- i 

 diately after it is protruded into the soil, and the young stem is elevated i 

 into the air, the root ceases to increase by the general distention of its tissue, | 

 and grows by the addition of new matter to its point. Hence the extreme ' 

 delicacy of the poiats of young roots, which, like all the newly formed parts ' 

 of vegetable matter, are extremely hygrometrical, absorbing water like a . 

 sponge, and hence are called spongelets or spongioles. Roots, from their 

 organic structure, are not permeable by water throughout their whole i 

 length, and it is only by means of the spongioles at the extremities of the 

 small fibres that they absorb nourishment. In general, the buds of plants 

 have a power of producing roots from their base, in a manner analogous to 

 seeds ; but much greater care is required on the part of the cultivator to 

 bring about this process, and with many plants it will not succeed. In 

 some, it may be effected by taking off a mature bud, and placing it in the 

 soil, like a seed ; but, in most plants, it is requisite to preserve a portion of 

 the stem along with the bud, as in striking Vines by buds ; in others it is re- 

 quisite to have a plate of the bark, with or without a portion of the soft 

 wood, as in propagating by budding on the living plant ; and in some a leaf 

 or leaves are requisite. Roots are also protruded from all parts of the stems 

 of some plants, as of most kinds of Willow ; and from the joints imme- 

 diately under the buds of most plants. On this last property depends the art 

 of propagating plants by cuttings inserted in the soil. In some plants cuttings 

 of the matured wood without leaves will emit roots : but ui many others, and 

 ilideed in most plants, roots are most freely produced from cuttings of unripe 

 or partially ripened wood, with the leaves on, and in a growing state ; and 

 even in those cases in which roots are produced from cuttings having no 

 leaves, if leaves are not speedily produced, the roots will decay, and the 

 cuttings will die. In short, the connexion between leaves and roots is as 

 intimate in cuttings, whether of stems, branches, or tubers, as it is between 

 the radicle and the plumule of the seed. A portion of the tuber of a Dahlia, 

 which has no bud, will produce roots ; and we have known these to live, and 

 the tuber to remain fresh, for upwards of a year, without leaves having been 

 protruded ; ultimately, however, the roots decayed, and the tubers soon after- 

 progress of the plant, in the growmg season, yet they continue to perform 

 their office even in the winter season, unless the soil which contains them 

 should be frozen. In this case they are much injured, and the spongioles are 

 w^ards rotted. Though roots are most active, and most essential to the 

 ruptured and destroyed ; but, when the growing season returns, new spon- 

 gioles are formed, commonly branching out from the fibres more numerously 

 than before. This result is sometimes produced by overpowerful liquid 

 manures poured on the roots of plants, which destroy the spongioles, and 

 cause tlie fibres to throw out a greater number. As plants absorb their food 

 chiefly, and almost entirely, by their roots, and as it has been proved that in 

 general the spongioles have no power of selection, it follows that plants may 



