FUNCTIONS OF PLANTS. 29 



manner, yet in general it will neither produce a callosity nor roots ; though 

 there are some exceptions, as in the willow tribe, and of these if the cutting is 

 prepared at both ends, and laid horizontally in the soil, then at both ends 

 callosities, and ultimately roots, will be formed. Hence a shoot of a willow 

 inserted in the ground at both ends, being bent for that purpose, so as to 

 form an arch, will root at both ends ; but this is a result that will happen in 

 the case of very few plants. 



111. The bark consists of two parts; the outer bark, formed entirely of 

 cellular matter, and resting on the liber or inner bark ; and the inner bark, 

 which consists partly of woody, and partly of cellular matter. The latter 

 ultimately becomes wood, and the former ultimately hardens, cracks, and 

 sometimes falls off. No wound in the outer bark can be healed or filled up, 

 but the reverse is the case with wounds in the alburnum. The wood in all 

 exogenous plants of the tree kind is distinguishable into the heart wood, or 

 that which is mature, and the soft wood or alburnum, which is wood in a 

 young and growing state. The heart wood is for the most part of a darker 

 colour than the soft or young wood, which is generally white, till by age its 

 tubes and vessels become thickened with matter deposited by the sap in its 

 ascent to the leaves, when it darkens in colour, at least in most trees. When 

 the sap absorbed by the spongioles enters the solid matter of the plant, it 

 passes upwards through the alburnum to the leaves, and being elaborated 

 there, it descends through the liber, communicating horizontally by means 

 of the medullary rays, with both the old and the young wood. Wherever 

 it penetrates, it deposits cellular matter, till at last in the old wood the pores 

 become completely filled up and hardened. 



112. The stems of all plants, and especially of exogenous trees, have, 

 beginning at the centre, pith, old wood, medullary rays, alburnum, liber or 

 inner bark, and outer bark. The medullary rays connect all the parts of 

 the section of a stem, or branches, horizontally ; and the ligneous fibres, 

 which penetrate all the parts excepting the pith, connect them longitudi- 

 nally, and complete the vegetable structure. In all plants whatever these 

 parts exist ; but in many herbaceous plants, especially annuals, and others of 

 short duration, they are not easily defined ; the wood, alburnum, and liber, 

 often appearing in one homogeneous body ; and the bark and the pith 

 only being quite distinct. The root stem differs from the stem above 

 ground in being without pith, without visible buds, and without an outer 

 bark : or at all events without a bark which cracks and decays, like that of 

 the stems and branches. There are exceptions in the case of some root 

 stocks of herbaceous plants, such as those of the Colchicum and the 

 Crocus ; but nevertheless this holds true in the underground stems or tubers 

 of the Potato, in the fasciculated tubercles of the Dahlia, and in most other 

 tuberous-rooted plants. 



113. Leaves are formed on the surface of stems at certain distances, and 

 in a certain order in each species of plant ; and at the base of the petiole of 

 each leaf, there is a bud either visible or latent ; in either case ready to be 

 called into action and produce a new stem, shoot, or branch, when the neces- 

 sary excitement is given. If the leaves are removed from a growing stem as 

 soon as they appear, no buds are formed in their axils ; or if the germs of 

 them have existed there, they die for want of the nourishment of the leaf. 

 Hence, by taking off every leaf as soon as it is protruded from an over 

 vigorous-growing shoot of the current year, that shoot may be prevented 



