76 FORMATION OF WOOD. 



new wood. The lark (fig. 114/c, « c, ^) protects the tender wood, 

 conveys the elaborated sap downwards from the leaves, and is the 

 part in which many valuable products, such as gum, tannin, and bitter 

 principles, are formed and deposited. The vascular bundles (fig. 114 

 f I, V p) convey the sap from the root to the leaves. This function 

 is carried on during the life of the plant by the annular vessels and 

 the pitted vessels, as well as other kinds of fibro- vascular tissue; but 

 in the fibres of the wood it ceases at a certain epoch, in consequence 

 of the tubes being filled up by secondary deposits, so as to form the 

 perfect wood, which gives strength and stability to the stem. 



Considerable difierences of opinion have arisen on the subject of 

 the formation of wood. All agree that it cannot be properly formed 

 unless the leaves are exposed to air and light, but physiologists differ 

 as to its mode of formation. Some say that it is produced in a hori- 

 zontal, others in a vertical direction. There seems to be no doubt 

 that the cambium cells perform an important part in the formation of 

 wood, and that their activity depends on the proper development of 

 leaves. These formative cells, although most easily detected in exo- 

 genous stems, are also present in the other forms of stems which have 

 been described. In Monocotyledonous stems these cambium cells are 

 situated in the centre of the bundles, and are concerned in the forma- 

 tion of the vascular tissue surrounding them. In woody Acotyle- 

 donous stems, as in Tree-ferns, these cells surround the vascular 

 bundles. After a certain time the cambium zones in these stems be- 

 come ligneous, and then the vascular bundles only grow at their ex- 

 tremity by means of unchanged cambium cells. In both these kinds 

 of stems the vascular bundles are limited, and the stems can only 

 increase laterally by ramifying or dividing dichotomously (fig. 137). 



Knight espoused what is called the vertical theory, considering the 

 wood as developed in a downward direction by the leaves, and in this 

 view he was supported by Petit-Thouars and Gaudichaud. These phy- 

 siologists maintain that there are two vascular systems in plants, an 

 ascending and descending; the one connected with the leaf-forma- 

 tion, or the spiral vessels ; the other connected with the production 

 of roots, or the ligneous fibres ; the cellular tissue being more especi- 

 ally concerned in horizontal development. Every bud is thus, accord- 

 ing to them, an embryo plant ficxed on the stem, sending leaves 

 upwards, and roots downwards. The dicotyledonous embryo was 

 supposed to be formed by two phytons (puroii, a plant) united, having 

 each an ascending and descending system of vessels, while the monoco- 

 tyledonous embryo was composed of one such phyton. In Palms, 

 Dracaenas, and other Endogenous stems, the peculiar manner in which 

 the fibres interlace (fig. 133, 2) favours the opinion that they are 

 developed like roots, by additions to their extremities ; and this is 

 also strengthened by the formation of adventitious or aerial roots, 



