300 PLANT MORPHOLOGY 



and look like roots, being much dissected into filiform divisions. The 

 ventral leaves are probably absorptive in function, true roots being want- 

 ing. Azolla has pinnately branched stems covered with minute, crowded, 

 overlapping leaves alternately arranged in two dorsal rows (Fig. 2545). 

 Each leaf has two lobes, the upper lobe floating and the lower one sub- 

 merged. The upper lobe contains cavities in which colonies of Anabaena 

 live. Long, slender rootlets arise from the lower side of the stem. 



In both genera the leaves are folded in the bud, not circinate as in the 

 Marsileaceae and Filicales. Each leaf has a single vascular bundle. The 

 stem develops by means of an apical cell with two cutting faces. The vas- 

 cular tissues are greatly reduced. The stem of Salvinia appears to be an 

 ectophloic siphonostele, that of Azolla an amphicribral protostele. 



Sporocarps. The sporocarps of the Salviniaceae are globular and thin- 

 walled, two or three occurring on a common stalk. In Salvinia they are 

 borne in groups at the base of the ventral leaves, arising as outgrowths 

 from them. In Azolla they are borne on lateral branches, chiefly in pairs, 

 on the lower lobes of the first leaves to appear. In both genera the sporo- 

 carps consist of an indusium enclosing a single sorus. The indusium 

 becomes hard at maturity, forming a nut-like structure. 



The sporocarps are of two kinds, both occurring on the same plant. 

 One contains only microsporangia and the other only megasporangia. In 

 Salvinia the two kinds of sporocarps are of the same size and contain many 

 sporangia, but in Azolla the megasporocarps are much smaller than the 

 microsporocarps and contain only one megasporangium. At first each 

 sporocarp of Azolla contains a young megasporangium with several 

 younger microsporangia at its base. Only one kind of sporangium con- 

 tinues its development, however, the other kind aborting (Fig. 255). In 

 both genera the microsporangia are borne on long, slender stalks arising 

 from a basal receptacle, while the megasporangia are short-stalked or 

 nearly sessile (Figs. 255 and 256A). As in the Marsileaceae, the sorus is 

 gradate. The sporangium wall is only one layer of cells thick and no 

 annulus is formed. 



As in the Marsileaceae and Filicales, the development of the sporangia 

 is leptosporangiate (Figs. 255 A and 2565). In the microsporangium of 

 both Salvinia and Azolla 16 spore mother cells are formed and all of these 

 give rise to tetrads, resulting in the formation of 64 microspores. In the 

 megasporangium of both genera, however, only 8 spore mother cells are 

 formed. These give rise to 32 megaspores, but only one of these matures, 

 the rest degenerating (Fig. 256Z)). The functional megaspore enlarges 

 until it finally completely fills the sporangium. It becomes very thick- 

 walled. In both genera the tapetum, which is cut off from the sporog- 

 enous tissue and consists of a single layer of cells, breaks down before the 



