EPIDERMIS. VENATION. 



87 



dermal cells generally, contain abundant chlorophyll-bodies and 

 starch. 



The guard-cells are capable of changing their form according 

 to the amount of light, the hygroscopic state of the atmosphere, 

 and other circumstances, and thus open or close the hole or stoma 

 between them. This action is of great importance in the physi- 

 ology of the plant (transpiration, p. 105). 



In Pteris cretica and flabellata the stomata develop as follows : A 

 young epidermal cell is divided by a curved partition into two cells, one of 

 which (Fig. 49) is called the initial cell of the stoma (i.c). This is again 

 divided by a curved partition into the mother-cell of the stoma (Fig. 49, 

 m.c) and a subsidiary cell (Fig. 49, s.c). 



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

 appears as a chink between them (Fig. 49, 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. 45, -46, 

 50). Above and below them, the mesophyll and epidermal cells 

 are generally thickened and prosenchymatous, in this way con- 

 tributing alike to the form and the function of the " vein." 



FIG. 50. (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 nbro-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 midrib (Fig. 50). 



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, 

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



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