18 



LEAVES. 



Fig. 11. 



Leaves. 



97. A leaf is composed of two systems, a fibro-vas- 

 cular, and a cellular system. 



98. The fibro-vascular is first formed and is composed 

 of spiral vessels and woody fibre derived from the me- 

 dullary sheath ; a fascicle of this tissue separates from 

 the sheath and directs itself outwards to the circum- 

 ference of the stem, and in doing so becomes enveloped 



in some cellular tissue it 

 pushes out from the bark, 

 when it makes its exit at 

 the circumference; this 

 bundle either expands at 

 once into a leaf which is 

 then called sessile, or the 

 bundle is yet further pro- 

 longed as a bundle for a 

 variable length before it' 

 expands into a lamina, 

 and the leaf is then called 

 petiolate, and the prolonged bundle a petiole. In the 

 first case the leaf consisting of one part, the lamina (a) ; 

 the second of two, the petiole (a), and lamina (b}. 



99. The further arrangement of structure in a leaf, 

 is the following : The fibro-vascular system is distri- 

 buted over the lamina by anastomosing ramifications 

 forming what are termed the veins; and there are two 

 systems of venation, the parallel and reticulated (fig. 11)> 

 the first occurring in leaves on monocotyledonous, 

 and the other in those of dicotyledonous stems. As 

 soon as the ramifications are complete upon the upper 

 surface of the leaf, they turn back, and beneath, to 

 ramify on the under surface, returning to the petiole, 

 and becoming continuous with the bark : now between 

 these different ramifications is contained the cellular or 

 parenchymatous system. 



100. The disposition of the parts of the parenchyma- 

 tous system is this : covering over the whole lamina of 

 the leaf is the epidermis ; beneath this is the true pa- 



