452 VASCULAR CRYPTOGAMS, 



Marsilia, except that all the leaves remain destitute of a lamina (Fig. 318); they 

 are long, conical, filiform, and at first rolled up spirally forwards. 



The Branching of Rhizocarps is similar to that of Ferns. Pringsheim states 

 that in Salvmia terminal branching never occurs ; new shoots arise, on the contrary, 

 exclusively from the basal part of the submerged leaves, each leaf of this description 

 forming a shoot on the side which faces the nearer aerial leaf; every branch pro- 

 duces at once a trimerous whorl of leaves. Strasburger, however, considers it 

 possible that these branches arise, as in Azolla (see ante\ from the stem itself 

 close to the leaves. The branching of the Marsiliaceae has been termed by Hanstein 

 axillary, a designation with which, however, I am unable to agree. The lateral 

 shoots have altogether the appearance of springing from the stem itself near to 

 the leaves, but at a later period they appear to be lateral and not axillary to the 

 leaves. As to their first origin, which has not yet been accurately ascertained, it 

 appears to be most natural to refer to the relative positions of these organs in 

 Azolla, where the lateral branches of the stem arise from cells lying anteriorly or 

 posteriorly or even dorsally to the origins of the leaves. 



The Growth of the roots of the Marsiliaceae and their monopodial branching 

 agree with that of Ferns and Equisetaceae in all important points. It has already 

 been mentioned that among the Salviniaceae, Salvmia itself is quite destitute of 

 roots, and that in Azolla the two ventral rows of cells of the stem give rise to 

 roots by the side of the lateral buds. The apical cell of the stem of these plants 

 forms segments in two rows, whereas that of the root is a three-sided pyramid 

 like that of the Marsiliaceae, Ferns, and Equisetaceae. A root-sheath is formed, 

 according to Strasburger, from the cells of the stem which lie over the endo- 

 genous mother-cell of the root of Azolla, which keeps pace with the growth of 

 the root and invests it during the whole of its existence. It is still more remarkable 

 that the root-cap of Azolla is formed from one single cell. From this cell two 

 layers of cells are derived which grow with the growth of the root and completely 

 invest it on all sides. 



The Sporocarps. The formation of the fructification shows even more evi- 

 dently than the structure of the vegetative organs, that the two families of the 

 Rhizocarpeae present considerable differences in certain particulars, which demand 

 separate consideration, as well as an agreement in essential characteristics. 



The SalviniacecB^ including the two genera Salvinia and Azolla, occupy with 

 respect to their fructification an intermediate position between the Ferns and the 

 Marsiliaceae. Their sporangia are enclosed in unilocular capsules, two or more 

 of which occur on the segments of the leaves (Fig. 319^, B). In Salvinia 

 these capsules are borne on the basal segments of the submerged leaves ; in Azolla 

 it is the external, downwardly directed segment of the deeply bisected leaf, and 

 moreover of the first leaf of each branch, which bears them. The apex of the 

 segment of the leaf which is to form the sporocarp grows out into a columella 

 (placenta) upon which the sporangia are developed, and at the same time an 

 annular wall arises round the base of the columella, and continues to grow until 

 it closes over its apex, thus forming the wall of the sporocarp by which the sorus 

 is completely enclosed. The sporocarp of the Salviniaceae closely resembles the 

 sorus of the Hymenophyllaceae, but differs from it in that the indusium of the 



