248 



THE SPORE-PRODUCING ORGANS 



[CH. 



by successive steps of simplification to the three-rowed stalk (/, <?,/), which 

 is the commonest of all in Leptosporangiate Ferns. But it again may pass 

 into still simpler structure («, q), till finally the stalk comes to consist of 

 but one row of cells (r), a state only found in the most advanced types 

 (compare Fig. 239, of Asplenium). A comparison of the figures shows that 

 structural complexity is not always determined directly by absolute size, 

 though on the average the thicker stalks show most cells in the transverse 

 section. On the other hand it is found that the stalks are roughly propor- 

 tional in transverse section to the bulk of the sporangia, and to the numerical 

 spore-output. (See Tables on pp. 262 — 263.) 



CD- 



.(P ©-^o 



F'ig. 243. Series of transverse sections of sporangial stalks, showing steps of pro- 

 gressive simplification. AH are approximately to the same scale. (X150.) 

 a = Gleichenia circinata ; b = Gkichenia dichotoma ; c = Mohria \ d,e~ Osnnmda ; 

 f—Matonia; g=Loxsofna; h, i= Thyrsopteris; j=Cibotmm culcita; k = Me- 

 taxya and Cheiropleuria\ l=Flatycerium; ni = Plagiogyria\ n, o~ Elapho- 

 glossum latifolium; p, q, r=Hypoderris Brownii. 



A comparison of {a, i, J) with {k, o, p) shows that the former are more 

 elaborate types, the latter simpler examples of the same tri-radiate segmenta- 

 tion, which corresponds to that usual in the sporangia of Leptosporangiate 

 Ferns. A comparison of {d) with {k) suggests a four-rowed structure, with 

 possibly a two-rowed initial segmentation, which is unusual in them. It has, 

 however, been shown above that such a segmentation actually exists, though 

 it has long been overlooked (Fig. 241). 



The length of the stalk varies greatly. In the Eusporangiatae it is 

 consistently short, or even absent. The Osmundaceae, Gleicheniaceae, Schi- 

 zaeaceae, Hymenophyllaceae, and Matonineae have spherical or pear-shaped 

 sporangia, with short massive stalks. But the Dicksonioid-Davallioid-Pterid 

 series, as also the Cyatheoid derivatives, illustrate an increase of length of stalk, 

 accompanied by decrease in its thickness; and this is commonly associated 



