ORGANS OF NUTRITION. 



147 



of the stem in the three great classes of plants respectively. We 

 shall now direct our attention more especially to that of the 

 leaves of Dicotyledonous Plants. 



The fibro-vascular system is in by far the majority of cases 

 double, that is, consisting of an upper layer which is in connex- 

 ion •svith the woody system of the stem (^fiy. 284,^,1?,/); and a 



Fig. 284. 



lower which is continuous with 

 the hber (/). The upper layer 

 therefore corresponds in its struc- 

 ture to the wood, and the lower 

 to the liber; hence the former is 

 composed of spiral and other ves- 

 sels and wood-cells; while the 

 latter consists essentially of hber- 

 cells, and laticiferous vessels. To 

 whatever extent the fibro-vascular 

 system may branch, each di\dsion 

 of the upper layer accurately cor- 

 responds at its apex Avith a similar 

 division of the lower. This double 

 layer of the fibro-vascular system 

 is readily seen in what are called 

 skeleton leaves, namely, those in 

 which the parenchyma between 



the veins has been destroyed by /-/g. 284. Fibro-vascular bundle pass- 

 maceration in water, or by other ing f'om a branch, b, of a Dicoty- 

 . ^11 1 • • ledonous riant into the petiole, p. 



means; thus the leaves lying in a „. Articulation between the pe 



damp ditch in the winter will af- 

 ford us good illustrations of these ; 

 similar leaves may be also arti- 

 ficially prepared by macerating for a sufficient time in water, &c. 



Although the fibro-vascular system of a leaf is in general 

 only double, instances do occur in which three layers of veins 

 have been found. Thus Lindley has described an example of 

 this kind in the leaves of Theophrasta Jnssicti. He says : — " In 

 this plant there are three layers of veins, of Avhich the middle is 

 much reticulated, but the upper and lower are far less so, their 

 fibres lying much more parallel with each other, and instead of 

 being apphed to the reticulations of the middle layer covering it 

 obliquely." 



The ramification of the fibro-vascular system in the lamina 

 of the leaf forming the veins, ribs, or nerves, will be described 

 presently under the head of venation. 



b. Parenchyma of Leaves. — This has been also termed diachyma 

 and viesophyllum. By it we understand the cellular tissue 

 situated between the epidermis of the upper and lower surfaces 

 of the leaf, and Avhich surrounds the ramifications of the fibro- 

 vascular system. It varies in amount in different leaves; in 

 L 2 



tide and the branch from which it 

 arises, t, t. Spiral vessels, v, v. An- 

 nular vessels. /, /. Wood-cells or 

 fibres. I, I. Liber-cells. 



