SECT. xiv. EXO GENO US STEMS. 405 



vascular tissue running from the roots upwards, and 

 passing at last into the leaves, where they form the ribs 

 or veins. Many of the higher forms of plants have fleshy 

 underground stems, as instances of which may be named 

 the Corm, as seen in the crocus andcolchicum, the Tuber, 

 as in the potato, the Rhizome, as in the fleshy rootstock 

 of Iris florentina, which yields the violet powder of the 

 shops, and indeed most bulbs may be considered as modi- 

 fications of stems, though they are more strictly analogous 

 to buds. The edible parts of the carrot, turnip, parsnip, 

 and radish are not stems, but highly developed succu- 

 lent roots, the unusual development of which is a di- 

 rect result of cultivation. 



A young tree, at the end of the first year of its growth, 

 has subterranean roots, with their branches and fibrils, 

 and an aerial stem, often more or less branched, formed of 

 bark, wood, and pith, with a few leaves at its extremity, 

 all exceedingly tender. Every succeeding year a new 

 cylinder of woody fibre and vascular tissue is formed 

 between the wood and the bark, both in the stem and 

 branches. It follows from this manner of growth, that 

 the stem of a tree, consisting of bark, wood, and pith, 

 is formed of a series of cylinders or extremely elongated 

 concentric cones closely united, so that a transverse 

 section exhibits a series of concentric circles or zones 

 from the surface of the bark to the central pith. The 

 structure of the branches is similar, but the number of 

 zones depends upon the age. Since all the tissues that 

 have been described are combined to form the organs of 

 nutrition and reproduction in a full-grown tree, it affords 

 the best general illustration of the organization of the 

 highest class of vegetables. 



Every part of a tree or plant, except the top of the 

 stigma and extremity of the roots, is covered by an ex- 

 tremely delicate film of cellulose, closely pressed down 

 upon one, two, or three layers of transparent colourless 



