526 



A TEXTBOOK OF THEORETICAL BOTANY 



and becomes flatter. A loop begins to bulge out at one side, becomes more 

 and more curved and detaches itself from the main bundle, first at one edge, 

 then at the other. This leaves a small gap in the curve of the main strand, 

 which is rapidly closed by its edges coming together. Meanwhile the loop 

 moves off to the base of the pinna, its curved face at first turning towards the 

 main petiole. The petiole of the pinna is twisted at right angles close to its 

 base so that its lamina lies in a line with the main petiole and thus the whole 

 frond is flat in appearance, though morphologically the plane of each pinna 

 is horizontal, not vertical as in other Ferns. 



The leaves are as a rule broadly bipinnate in the mature state, though 

 the youngest leaves in the sporeling state are ternate and the mature form 

 is only gradually acquired. Each pinnule is attached by a short, narrow 

 stalk and has a single midrib with pinnate veins on both sides, which do not 



-««i?j^r^ 



Fig. 524. — Osmunda regalis. Fertile frond showing 

 upper sporangiferous pinnae and lower 

 vegetative ones. 



anastomose. The structure of the sterile lamina agrees with the ordinary 

 Dryopteris type. There is no palisade layer, though the mesophyll is denser 

 towards the upper side and more lacunar towards the under side, on which 

 are numerous stomata. 



The pinnae are set more or less regularly in opposite pairs. The upper- 

 most half-dozen pairs are usually wholly fertile (Fig. 524), but intermediate 

 stages occur in the pinnae immediately below this level, which may be 

 partly fertile and partly sterile. Among their pinnules there are often to 

 be found some in which the lamina is reduced but which bear only the 

 rudiments of abortive sporangia. 



The wholly fertile pinnules are reduced to little more than the width of 

 their midribs and bear two naked, marginal masses of sporangia which, when 

 mature, completely conceal the pinnules and give the impression of covering 

 their entire surface. This is an important distinction between Osmunda and 



