130 



OEGANOGRAPHT. 



Fig. 250. 



layer therefore corresponds in its structure to the wood, and the 

 lower to the liber; hence the former is composed of spiral and 

 pitted vessels in perennial plants, 

 and of spiral and annular or some 

 other vessels in herbaceous plants, 

 and also in all cases of wood- 

 cells, besides the vessels ; while 

 the latter consists essentially of 

 liber-cells and latieiferous vessels. 

 To whatever extent the fibro-vas- 

 cular system may branch, each divi- 

 sion of the upper layer accurately 

 corresponds at its apex with 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 

 maceration in water, or by other 



means ; thus the leaves lyin^ in a 



^Jfl'. 250. Fibro-vascular bundle pass- i „ ,Ti.„i, •„ 4.1 -^^.^^ ™:n „^ 



ingfrom a branch, 6, of an herba- damp dltch m the Winter Will af- 

 ceous Dicotyledonous Plant into ford US ffood illustrations of these ; 

 the petiole, «. a. Articulation be- j • -i i 1. 1 



tween the petiole and the branch and Similar leaves may be also 

 from which it arises, t, t. Spiral ves- artificially prepared by macerating 

 sels. 1;,^. Annular vessels./,/. Wood- ,•■ r J^ • 1. J • -j 



cells or fibres. I, I. Liber-cells. them for a suflicient time in acidu- 

 lated water, and in other ways. 

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

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

 have been found, as in the leaves of Theopkrasta Jtissiai. The 

 ramification of the fibro-vascular system in the lamina of the 

 leaf forming the veins or nerv^es, will be described presently un- 

 der the head of venation. 



b. Parenchyma. — By this we understand the parenchymatous 

 tissue which is situated between the epidermis of the upper and 

 lower surfaces of the leaf, and which surrounds the ramifications 

 of the fibro-vascular system (^fig. 252, fv). It varies in amount 

 in different leaves ; thus in ordinary leaves it is moderately 

 developed, and the leaves are then thin and flattened ; while in 

 other leaves it is formed in large quantities, when they become 

 thick and fleshy, and are termed succulent. In ordinary flat 

 leaves all the cells composing the parenchyma are commonly 

 green from containing chlorophyll ; but in succulent leaves the 

 ' cells in the centre of the parenchyma are usually colourless. 



The parenchyma also varies in the form and arrangement of 

 its component cells in diiFerent parts of the same leaf; thus in 

 ordinary flat leaves we find beneath the epidermis of the upper 

 surface one (Jig. 251), two, or three layers of oblong blunt cells 



