90 



THE BIOLOGY OF A PLANT. 



were discovered in 1648 by Csesius. They arise inPteris upon a 

 longitudinal thickening of tissue situated on the under side of the 

 leaflets near their edges, and including a marginal anastomosis 

 of the veins. This swelling is known as the receptacle. Hairs 

 are not uncommon upon the under side of the leaf, and some 

 are found upon or near the receptacle. On the latter arise struc- 

 tures at first superficially similar to hairs, which become enlarged 



B 



o.i. 



FIG. 52. (From Luerssen, after Burck.) Indusia and receptacle of Pteris aquiUna ; B 

 (diagrammatic), seen from below; A, in the section of the edge of a leaflet, o.i, outei 

 (false) indusiuni; /./, inner (true) indusium; r, receptacle; s, young sporangia. 



at the tip, and finally develop into the sporangia. Meanwhile 

 the ed<re of the leaflet is bent down and under so as to make 



O 



a longitudinal band of thin tissue composed of epidermis known 

 as the outer veil or indusiuni (Fig. 52, J3, o.i}. A similar 

 thin sheet of epidermis grows down from the under side of 

 the leaf, and passing outwards to meet the former, constitutes 

 the inner veil or true indusium (Fig. 52, B ', i. i). 



In the V-shaped space thus formed the sporangia are developed 



