FI LI CINE A E. —HE TER OSPOR US FI LI CINE A E. 



239 



thick layer over its apex ; this frothy mucilage, which eventually hardens, is the 

 substance which forms the thick envelope, the episporium (formerly called the ex- 

 osporium), round the ripe macrospore (Fig. 184 ^); it splits as it forms into three 

 lobes above the apex of the spore, and from the space between these lobes the 

 prothallium afterwards protrudes. Strasburger proved some time since the existence 

 of this hardened frothy slime in both kinds of sporangia in Azolla, and there it 

 assumes very striking forms ; in the microsporangia it looks like a large-celled tissue, 

 and collects into from two to eight portions (massu/ae) quite distinct from one 

 another, each of which encloses a number of microspores; in some species, as 

 A. filiculoides and A. caroliniana, these massulae have on their surface hair-like 



Fig. 194. Satvinia iiatans. A transverse section of the stem, bearing a whorl ; / aerial leaves, to submerged Jeaf 

 with several teeth, b sporocarps on the teeth. B longitudinal section through three fertile teeth of a submerged leaf; 

 a a sporocarp with macrosporangia, ii two similar ones with microsporangia. C transverse section of a sporocarp with 

 microsporangia mi. D transverse section of the aerial leaf; htc hairs of the under side, ho hairs of the upper side, 

 ep epidermis, / air-cavities, the shaded ones showing the vertical walls of the tissue in the background. E cells of a 





after the 



been 



ted 



glycerine. A natural size. 



appendages barbed at their upper end {glochidia), by means of which when they have 

 issued from the sporangia and are floating in the water they anchor themselves to the 

 macrospores which are also floating about there. The roundish macrospore of 

 Azolla, which does not nearly fill the sporangium, is completely covered with a very 

 thick verrucose layer of hardened frothy mucilage, which projects high above the 



