284 BOTANY 



The Leaf. The leaves in Salvinia are arranged in alternating whorls of 

 three, so that there are four rows of dorsal leaves and two of ventral ones. In 

 both Salvinia and Azolla leafless segments alternate with leaf-bearing ones, thus 

 dividing the stem into nodes and internodes. 



The dorsal leaf in Salvinia is composed of large air-chambers, arranged in 

 two layers. Between these are single layers of chlorophyll-bearing cells, which 

 do not differ essentially from the epidermal cells. In Azolla the ventral lobe of 

 each leaf consists of a single layer of cells, but the dorsal lobe is composed of 

 elongated loosely placed mesophyll, bounded by the epidermis. There is always 

 found in the dorsal lobe of the leaf a large cavity, communicating with the exte- 

 rior, and containing a colony of blue-green Schizophytes (Anabcena azoll<R). 

 The Anabsena grows about the apex of the shoot, and a filament creeps into the 

 cavity of each young leaf as soon as it is formed. Stomata are developed upon 

 the upper epidermis of the leaf in Azolla. 



The leaf in Salvinia grows from a two-sided apical cell, as in the typical 

 Ferns, but this is not the case in Azolla. 



Root. The roots in Azolla arise from external cells, instead of endogenously. 

 The first outer root cap-segment develops into a sheath, which encloses the root, 

 and only one other cap-segment is formed. Otherwise the root is like that of 

 the typical Ferns. 



The Sporangium. The sporangia (Fig. 249), which in their devel- 

 opment correspond to those of the other Leptosporangiatae, arise 

 from special leaf-segments. The sori are borne upon the ventral 

 leaf in Salvinia, and replace the ventral half of the leaf in Azolla. 

 The sorus is completely enclosed by the indusium, which is cup- 

 shaped at first, but finally becomes globular and completely closed 

 at the top. In their position and the form of the indusium, the 

 sporangia of the Salviniacese suggest the Hymenophyllaceae. The 

 formation of the indusium about the single macrosporangium of 

 Azolla strikingly resembles the development of the integument 

 about an ovule. 



The macrosporangia and microsporangia are in separate sori. 

 The former are less numerous, and in Azolla reduced to a single 

 one. The sporangia arise from a central receptacle, or placenta, and 

 in Salvinia the microsporangia are borne at the ends of the divisions 

 of a branching stalk. 



The early divisions in all the sporangia are alike. From the central tetrahedral 

 archesporium are cut off the tapetal cells, as in the typical Ferns, and the central 

 cell then divides into eight, or in the microsporangium into sixteen sporogenous 

 cells, all of which divide. In the microsporangium, all of the sixty-four young 

 spores develop ; in the macrosporangium, a single spore of one tetrad grows 

 more rapidly than the others, and finally occupies the whole of the sporangium, 

 destroying the other spores. The nucleated protoplasm derived from the disin- 

 tegrated tapetum is not all used up in the development of the macrospore, but 

 part of it persists in the form of the "peculiar episporic appendages, which are 

 especially conspicuous in Azolla (Fig. 251). In the microsporangia the spores 

 do not nearly fill the cavity, but are imbedded in a foamy mass of hardened 

 protoplasm, derived from the tapetum. This is divided into several parts, or 

 "Massulae," in Azolla, and upon the massulse are developed curious anchorlike 



