VEGE TA Tl VE REP ROD VCTION. 



195 



Each cluster may be protected by a special outgrowth from 



the cells in its neighborhood (figs. 149, 150). Each spore 



case consists of a stalk expanding above 



into a body composed of a single outer layer, 



enclosing at maturity the loose spores (fig. 



236). 



276. Spore leaves. — In many of the ferns 

 the leaves which produce spore cases are 

 not different from the foliage leaves. In 

 others, certain leaves are so specialized for 

 bearing the spore cases that they abandon 

 their nutritive work in part or entirely. To fig. r4q.-A leaflet of 

 such specialized leaves the name spore 3.ier^{Aspiditi.vi) 



^ ^ seen from the back. 



leaves is applied. ^'s*" clusters of 



^^ spore cases are 



shown, each cov- 

 ered by its own in- 

 276a. Differentiation of sp res — Ar.-.ong higher dusium, i. Mag- 

 fern worts the spores are of two sizes: large ones, Aftef Sachs '^'"' ~ 

 known as megaspores, and much smaller ones known 

 as microspores (fig. 151). Each kind, when it germinates, produces a, 

 sexual plant. The megaspores give rise to plants bearing female organs 

 only, the microspores to those bearing male organs only. A similar 

 separation of sexes in the gametophytes frequently occurs when the spores 



Fig. 150. — Vertical section through the leaflet shown in fig. 149, passing through the 

 center of a spore-case cluster, f, ventral epidermis ; e' ^ dorsal epidermis ; between 

 them the mesophyll, showing 3 veins cut across ; over the central one is a cushion of 

 tissue from whose surface arise the stalked spore cases J, s, z, z", the indusium. Mag- 

 nified about 30 diam. — After Sachs. 



are equal in size, as in Marchantia and horsetails, but it always occurs 

 when they are unequal. A corresponding difference in size is often, found 



