INTERNAL STRUCTURE. 67 



Fif. 124. Here is no differentiation whatever into stem and 



O 



foliage ; but the expanded floating body which serves for both 

 must be counted as stem developed horizontally into a flat plate, 

 for it produces a root from the under surface and a flower from 

 the edge. This simplification is common in some orders of 

 Cryptogamous plants ; and such a bod}', which answers both for 

 stem and foliage, is termed a FROND, from the Latin frons, which 

 means either leaf or leafy bough. In some species of Lemna 

 the frond is thickened or plano-convex : in Wolffia, the simplest 

 and smallest of phaenogamous plants, it is a globular green mass, 

 seldom much larger than the head of a pin, wholly destitute of 

 root, propagated by proliferous budding from one side, and from 

 within the top producing a flower or pair of flowers. 



3. INTERNAL STRUCTURE. 



130. The investigation of the intimate structure of the stem, 

 as of the other organs, belongs to vegetable anatomy or histology 

 (treated in Part II.) ; but the general outlines of structure, so 

 far as is requisite to the explanation of what is visible to the 

 naked eye, should be here explained. 



131. The stems of phsenogamous plants anatomically consist 

 of two general elements, the cellular and the woody ; the former 

 exemplified in the commoner stems by the pith and outer bark, the 

 latter by the wood. Both are equally composed of cells, or origi- 



124" 



nate as such ; but those which form the woody system of the stem 

 mainly undergo, at a very early period, transformation into tubes, 

 some of which are of such small calibre that their common name 

 of fibres is not inappropriate ; others, of larger size or ampler 

 calibre, take the name of ducts or vessels. The latter are almost 



FIG. 124 a . A magnified slice of a portion of the flower-stalk of Richardia jEthiopica 

 (the so-called Calla Lily), transverse with some longitudinal view : mainly parenchyma, 

 the cells huilt up so as to leave comparatively large vacancies (intercellular spaces or 

 air-passages) ; near the centre a cross-section of a fibro-vascular bundle, and next the 

 margin or rind some liner ones. 



