394 MOSSES AND FERNS chap. 



- * 1^ — — • \ 



leaves doubtless replace the roots. The leaves in Salvinia are 

 arranged in alternating whorls of three, corresponding to the 

 nodes, and this arrangement accounts for the six rows of leaves 

 previously referred to. 



The mature stem shows a central concentric vascular bundle 

 (Fig. 203, E, F), whose tracheary tissue is somewhat more com- 

 pact and the tracheae larger in Azolla. This is surrounded by 

 a definite endodermis and one or two layers of larger paren- 

 chyma cells, and radiating from the latter are plates of cells 

 separated by large air-spaces, and connecting the central tissue 

 with the epidermis (Fig. 203, E). 



The lateral branches arise in acropetal order, but apparently 

 not always at equal intervals. Their development is a repetition 

 of that of the main axis. Like the branches, the roots in Azolla 

 arise acropetally, and their number is very much less than the 

 leaves. They arise from superficial cells and follow exactly in 

 their development the primary root of the embryo. The inner 

 layer of cells of the sheath, however, in these later roots be- 

 comes disorganised, and there is a space between this and the 

 root itself. A single root-cap segment only is formed subse- 

 quent to the primary one from which the sheath forms, and this 

 secondary cap segment undergoes division but once by periclinal 

 walls (Fig. 204, C). 



TJie Spo7^angia 



The sporangia in both genera are contained in a so-called 

 sporocarp, which is really a highly-developed indusium. These 

 sporocarps always arise as outgrowths of the leaves, in Salvinia 

 from the submersed leaves, in Azolla from the ventral lobes. In 

 Salvinia several are formed together (Fig. 196, C), in Azolla 

 two, except in A. Nilotica, where there are four. Each 

 sporocarp represents the indusiate sorus of a homosporous 

 Fern. 



In Azolla filiculoides these sori arise, as Strasburger '^ showed, 

 from the ventral lobe of the lowest leaf of a branch. My own 

 observations in regard to the origin differ slightly from his in 

 one respect. Instead of only a portion of the ventral lobe 

 going to form the sori, the whole lobe is devoted to the forma- 

 tion of these, and the involucre which surrounds them is the 

 reduced dorsal lobe of the leaf, and not part of the ventral one. 



^ Strasburger (6), p. 52. 



