XLViii] SALVINIA 261 



In their distal marginal position upon the leaf-segments the sori of the 

 Salviniaceae correspond to thoseof the Hymenophyllaceae, or to Thyrsopteris. 

 This position was observed by Pringsheim for Salvinia, but it has been ex- 

 plained in greater detail by Von Goebel {I.e. pp. 1060, 1 193). Here the dorsal 

 leaves act as floats and serve nutrition: the sori are borne upon the ventral, 

 submerged leaves, which arise late on the young plant, and are richly branched. 

 They maybe designated the sporophylls,and the distinct male and female sori 



(iii) 



(iv) 



Fig. 754. Azolla filiciiloides Lam. (i) Longitudinal section through a megasorus. 

 yi^=indusium: i1/rt = megasporangium : /"= placenta : enlarged. Above the megaspo- 

 rangium Anahaena-\}nx&'<k^% are seen: below it are rudiments of abortive raicrosporangia. 

 (ii) I = habit of a shoot seen from above, enlarged. II = transverse section through a bud : 

 three pairs of leaves are marked o, u, etc., being the upper and lower lobes of the re- 

 spective leaves. The position of the palisade parenchyma is shown by shading on the 

 upper leaves, (iii) I = sporophyll dissected free, seen superficially: Crupper lobe: 

 /'= rudiment of its flange: S\, 6'2 = ™diments of megasporangia : Id\, 7^2 their indusia. 

 11 = transverse section through two leaves, the left sterile, Oi its upper, its lower 

 lobe. Right a fertile leaf focused in upper and lower planes, the latter dotted. 6* = upper 

 lobe, /^=its flange covering the two megasori. 111 = the lower lobe dissected free: it 

 is wholly used up m the formation of two megasori : the indusia appear as circular walls, 

 (iv) Sporophyll of Azolla JiHculoides, flattened out. To the left are two megasori, to 

 the right the upper lobe : i^=the flange-like outgrowth, below the mucilage-cavities are 

 visible. (After Von Goebel.) 



are borne in bunches of 3 to 8, each terminal upon an abbreviated leaf-segment 

 (compare Luerssen, I.e. Fig. 185). Their exact position in Azolla, which is 

 different in detail from that in Salvinia, has lately been re-examined by 

 Von Goebel {I.e. pp. 1062, 11 32). The leaves are arranged in two rows, and 

 each leaf is two-lobed. The upper lobe is nutritive, and harbours the well- 

 known Anabaena-co\ox\y: the lower consists normally for the most part of 

 a single cell-layer (Fig. 754, (iii) ii). It is this lobe which bears the pairs of 



