FUNCTIONS OF PLANTS. 27 



he poisoned in the same manner as they are nourished ; and hence it has 

 been found that solutions of opium, mercury, arsenic, and even common 

 salt, presented to the roots of plants, will destroy their vital powers. In 

 general the roots of plants are not furnished with buds, and hence roots 

 cannot be used in propagation in the same manner as branches ; nevertheless, 

 there are numerous exceptions, and some extensive orders of plants, such 

 as the Rosaceae, Campanulaceae, Cruciferse, and some of the AmentacecR, 

 have roots abounding in adventitious buds, and if these roots are cut into por- 

 tions, and planted in the soil with the part of the root which was next the 

 stem uppermost, and their points exposed to the air, or very slightly covered, 

 they will produce plants. This, however, is never the case with the roots of 

 annuals or biennials; and hence, in Cruciferae, while the common Sea-kale 

 produces buds in abundance from the cuttings of the roots, the same thing 

 never takes place in the common Cabbage. The nature of plants in this 

 respect is very different ; for while the fasciculated tubercles of the Dah- 

 lia, if deprived of the plate which produces the buds, have no power of 

 originating fresh buds, yet the tubers of the common Paeony so treated, 

 produce them freely. 



104. Every plant contains nitrogen in its albumen and gluten, and it has 

 been found that this elementary principle abounds in a particular manner in 

 the spongioles of the roots, and in all the newly-formed parts of plants, and 

 that those seeds germinate the earliest which contain the largest quantity of 

 nitrogen. Hence the great value of animal manures to plants, all of which 

 contain nitrogen ; but especially those of carnivorous animals. (Lieb. p. 190.) 



105. The stem of plants is not protruded so early as the root ; but as soon 

 as the latter is in a state of action, and has penetrated a few inches into the 

 soil, the seed-leaves appear above the surface, and from the centre of these 

 is originated the stem. Both the roots and stems of plants, when first 

 originated from seed, are perpendicular to the earth's surface, or in other 

 words, they extend in the direction of radii from the earth's centre. The 

 root, which penetrates downwards, always avoids light, and the stem, which 

 rises upwards, as constantly seeks the light, and avoids darkness. There 

 are some apparent exceptions to this law ; as, for example, in the Mistletoe, 

 the seeds of which, when deposited on the underside of a branch, send their 

 radicles upwards, and their stem downwards ; and this may perhaps also be 

 said of some orchideous epiphytes ; but in general, few laws are so universal 

 as that of the ascending and descending axis of a plant being always in the 

 direction of a radiating line from the centre of the earth. 



106. The stem at first is a mere point, scarcely so large as to be recognised 

 as a bud ; but as soon as it feels the effect of the nutriment impelled into 

 it by the growing root, it becomes developed, enlarged, furnished with leaves, 

 and solidified. From being a small portion of cellular tissue, possessing 

 neither strength nor tenacity, it becomes by the formation of woody matter, 

 a slender rod or shoot, sufficiently firm and tough to require an effort to sepa- 

 rate it from the root ; and, in a short time, it adheres to the latter so firmly 

 as when drawn up forcibly to pull the entire plant out of the soil. 



107. Before the formation of leaves on the stem, it is quite succulent, and 

 without woody fibre; but as soon as the leaves appear, woody matter is de- 

 posited in the form of tubes of extreme fineness, which originating in the 

 leaves, pass downwards through the cellular tissue, and are incorporated with 

 it, so as to add to its bulk, strength, and flexibility. The first woody matter 



