FORMATION OF WOOD. 59 



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. 



110. Considerable differences 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 deposition. Some say that it is deposited 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, appear also to be present in the other forms of stems 

 which have been described. 



111. The early physiologists made experiments on exogenous stems, 

 as being most easily procured. They espoused the horizontal theory 

 of deposition, and disputed as to the formation of cambium ; some 

 maintaining that it was formed by the cells of the bark ; others, by 

 the central cells of the stem ; and others, by both united. Duhamel, 

 by putting silver plates between the bark and wood, and Dr. Hope, 

 by detaching portions of bark, endeavoured to show that the bark 

 alone was concerned in the formation of wood ; while Decandolle 

 and others were led to the conclusion, that both were concerned in 

 the process of forming cambium, by means of which a layer of liber 

 and a layer of wood was annually produced. 



112. 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 is supported by Petit-Thouars and Gaudichaud. These 

 physiologists 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 woody fibres the cellular tissue being more especially 

 concerned in horizontal development. Every bud is thus, according 

 to them, an embryo plant fixed on the stem, sending leaves upwards, 

 and roots downwards. In Palms, Dracaenas, and other Endogenous 

 stems, the peculiar manner in which the woody fibres interlace (fig. 

 114, 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, which burst through different 

 parts of the stem in Screw-pines (fig. 115, 2), in the Banyan, and in the 

 Fig tribe in general. In Vellozias and Tree Ferns, the surface of the 

 stem is often covered with thin roots, protruding at various parts, and 

 becoming so incorporated with the stem as to appear to be a part of 

 it. In the Tree Fern, represented hi fig. 116, the lower part of the 

 stem is enlarged in a remarkable degree by these fibres, so as to give 

 it a conical form. In Exogenous stems, when ligatures are put round 



