FORMATION OF WOOD. TV 



■which burst through dififerent parts of the stem in Palms, Screw- 

 pines (fig. 134, 2), the Banyan, and in the Fig tribe generally. 

 In Vellozias and Tree Ferns, the surface of the stem is often covered 

 ■with thin roots, protruding at various parts, and becoming so incor- 

 porated ■with the stem as to appear to be a part of it. In the Tree- 

 Fern, represented in fig. 135, the lower part of the ^tem 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 the stem, and ■when 

 portions of bark are removed, a swelling takes place above the parts 

 ■where the injury has been inflicted, thus apparently proving that the 

 new matter is developed from above downwards. 



Gaudichaud endeavours to account for various anomalous forms of 

 stems (figs. 123-126), by considering them as depending on the 

 arrangement of the leaves, and on the mode in which the woody 

 fibres are sent do'wn from them. Thus, the four secondary masses 

 surrounding the central one in the stem of Galycanthus floridus are 

 traced to four vascular bundles from the leaves, penetrating the cellu- 

 lar tissue of the bark, distinct from the central wood and from each 

 other, except at the nodes, where the cross bundles unite them so as 

 to form a ring round the central mass. New fibres are formed on the 

 inner side of these bundles, and by degrees they assume a orescentic 

 shape, while the horns of the crescent ultimately unite on the outer 

 side (centrifugally), and enclose a portion of the bark, which thus forms 

 a kind of spurious excentric pith, with numerous woody layers on the 

 inside, and a smaller number on the outside.' Again, in Brazilian 

 Sapindacese (fig. 124), with five, seven, nine, or ten woody masses, 

 the same thing is said to occur, with this difference, that the pith of 

 each of the masses is derived from the original medullary centre, por- 

 tions of which are enclosed by the vascular bundles in a centripetal 

 manner, or from without, inwards. 



Treviranus states that the fibrous and vascular bundles descending 

 from the leaves are destined in general to unite around a common 

 centre, but that they retain a certain degree of independence, and 

 may be developed separately in some instances, gi^ving rise to ano- 

 malous fasciculated stems. 



Gardner, from an examination of Brazilian Palms, adopts the 

 vertical the(wy. It is, however, opposed by most vegetable physio- 

 logists, who consider the development of the vascular bundles as 

 proceeding from below upwards; in Dicotyledons, by peripherical 

 production of woody and vascular tissue from cambium cells ; and in 

 Monocotyledons, by a definite formation of woody and vascular 

 bundles by means of terminal buds ; the hardening of the stem de- 

 pending on the interstitial changes which take place afterwards in the 

 woody fibres. 



All physiologists agree in believing that the formation of woody 



