238 MORPHOLOGY. 



most distinct when the vascular bundles are closed; but we see 

 plainly enough, in Mammillaria or Melocactus> the bundles going to 

 the basis of the lowest leaves, coming out from the most internal 

 parts of the wood, and running past in a curve across all the subse- 

 quently developed parts. 



At the point where a leaf is given off, in the Dicotyledons always, 

 in Monocotyledons at most indistinctly, often not at all, several 

 neighbouring vascular bundles become applied together, to form a 

 loop (ansa\ from the circumference of which pass off the vascular 

 bundles of the leaf and the axillary buds. 



Wood is formed from the unlimited vascular bundles of Dicoty- 

 ledons, by their longer duration. The new cells originating be- 

 tween them, which correspond to the medullary rays, become 

 again parenchymatous or medullary-ray cells, for these latter 

 becoming compressed at the sides, by the enlargement of the vas- 

 cular bundles, deviate somewhat in form from the common paren- 

 chymatous cells. 



Frequently, however, one or more cells remain as parenchy- 

 matous cells, and so begin to form medullary rays in the midst of the 

 wood (called small medullary rays), which sometimes go on develop- 

 ing for a long time, and sometimes cease at an early period. The 

 wood does not generally grow uniformly continuously ; in those 

 parts, especially where, owing to climatal conditions, an alterna- 

 tion occurs every year between the active and dormant periods of 

 vegetation, more vessels are formed at the beginning of the period of 

 vegetation, and at its close wood-cells which are narrower and have 

 stronger and thicker walls. By this means a division of the wood 

 into more or less concentric hollow cylinders is occasioned, or 

 those circles on the transverse section which are commonly termed 

 annular rings. 



In the Dicotyledons, where the vascular bundles are situated in 

 several circles, they gradually unite together as they are succes- 

 sively developed, and form a close mass of wood, in which run 

 then the separate vertical cords of the separate vascular bundles 

 belonging to the cambium, giving the wood a peculiar appearance, 

 which is beautifully exhibited in the species of the Pisonia. 



There is but little that can be said generally of the composition of the 

 axis from the separate forms of the elementary parts and of the tissue ; 

 all forms occur in the stem, and it is only in the case of individual 

 groups of plants that we meet with certain forms or combinations 

 specially or exclusively. Thus the Labiate are distinguished by having 

 a square stem, the margins of which are formed by four strips of distinctly 

 characterised cortical substance. The majority of the Euphorbiacece 

 have milk-vessels, as the Asclepiadacece and Apocynacece are provided 

 with their peculiar intermediate form between milk-vessels and liber- 

 cells. Nepenthes is distinguished by having elongated spiral cells, which 

 occur scattered in large numbers over every part of the stem. The 

 distinction between pith and bark is not an universal essential charac- 

 teristic for plants, as may be seen by the innumerable transition stages 

 occurring between them. The two continually merge into one another. 



