VENATION. 129 



divided by a curved partition into the mother-cell of the stoma (Fig. 61, 

 m.c) and a subsidiary cell (Fig. 61. s.c). 



The mother-cell is then bisected into the two guard-cells, and the stoma 

 appears as a chink between them (Fig. 61, B). 



The veins are the fibres or threads which constitute the 

 framework of the leaf. Each consists, essentially, of a small 

 fibro-vascular bundle branching from that of the midrib (Figs. 

 57, 58, 62). Above and below them the mesophyll and epi- 

 dermal cells are generally thickened and prosenchymatous, in this 

 way contributing alike to the form and the function of the 



' ' vein. 



FIG. 62. (After Luerssen.) Venation of a leaflet of Pteris aquilina. 



Their arrangement (veining or venation) is definite, and depends on the 

 mode of branching of the fibro-vascular strand which constitutes the prin- 

 cipal part of the midrib. Secondary strands (nerves) proceed from this at 

 an acute angle, then turn somewhat abruptly towards the edge of the 

 leaflet (or lobe), making an arch which is convex towards the distal ex- 

 tremity of the midrib (Fig. 62). 



From this point, after branching once or twice, the delicate veins run 

 parallel to each other to the edge of the leaflet, where they join one another 

 or anastomose. This form of venation is known as Nervatio Neuropteri- 

 dis, and is more easily seen in the leaf of Osmunda regalis (cf. Luerssen, 

 RabenhorsVs Kryptogamen-Flora (1884), III., s. 12). 



