OF THE STEM OR ASCEXDIXG AXIS. 747 



and other nutritious substances in a state of solution. As the 

 parts increase in age the pith loses its colour, becomes dry, and 

 is generally more or less destroyed. It may possibly serve the 

 temporary purpose of nourishing the parts which surround it 

 when they are in a young state. The pith also, in some cases, 

 acts as a reservoir for the secretions of the plant. 



2. The Wood. — The wood, when in a young and pervious con- 

 dition, is the main agent by which the crude sap is conveyed 

 upwards to the external organs to be aerated and elaborated, 

 but whether the passage is primarily by the vessels or the wood- 

 cells is disputed. (See pp. 739 and 740.) The vessels of the 

 medullary sheath and of other parts are carriers of sap in the 

 young plant, but when old contain air. (See p. 740.) As the wood 

 increases in age, the tissues of which it is composed become 

 filled with deposits and secretions by which they are hardened 

 and solidified, and in this manner the stem acquires strength and 

 firmness, but the tissues are no longer physiologically active, and 

 are in fact useless as carriers of sap. On the outside of the 

 young wood, but organically connected with it and with the liber 

 of Dicotyledons, is the vitally active layer of cells called the 

 cambium layer, from which are annually formed new layers of 

 wood and inner bark. The cells of the cambium la3-er are filled 

 in the spring, and at other seasons when growth takes place, 

 with elaborated sap, or that sap which contains all the materials 

 necessary for the development of new structiu'es. Great differ- 

 ences of opinion exist amongst botanists as to the exact manner 

 in which wood is deposited, but they are nearly all agreed that 

 the materials from which it is formed are elaborated in the 

 leaves, that -without leaves there can be no additions to it, and 

 that in proportion to their amount so will be the thickness of 

 the wood.* It is necessary, therefore, that the process of 

 pruning timber trees should be carefully conducted, and that 

 they should be placed at proper intervals, in order that they 



* Herbert Spencer believes that intermittent mechanical 

 strains, such as those produced by the wind, are the sole cause 

 of the formation of wood, which is developed to resist the strains. 

 His experiments were anticipated by Knight so far back as 

 1803; and his results must be taken with modification. It is 

 probably true that such a conservative formation of wood does 

 occur to meet unusual strains ; but the want of cori^espondence 

 in nature between great exposure to such strains and large 

 deposit of wood, and the numerous examples of great wood- 

 formation in ligneous twiners and nailed-up trees, must prevent 

 us from considering it an all-sufficient explanation. In the cases 

 where no strains can have occurred, "the natural selection of 

 variations can alone have operated" to form wood, according to 

 Mr. Spencer. 



