PRINCIPAL GROUPS OF PLANTS. 101 



The plants have for the most part well-differentiated stems 

 and leaves, and represent the sporophyte or asexual generation. 

 The sporophyte produces sporophylls which are of two kinds, 

 namely, megasporophylls and microsporophylls. The megasporo- 

 phylls bear small ellipsoidal bodies known as ovules, which develop 

 into seeds. The megasporangium is not separate and distinct in 

 the spermophytes as it is in Selaginella, but is embedded within an 

 ovule and corresponds to that part of the ovule known as the 

 nucellus. The nucellus encloses the embryo-sac, which is regarded 

 as a megaspore (Figs. 70, 71, and 81). Each megasporangium 

 (nucellus) therefore contains but a single megaspore, whereas in 

 Selaginella the megasporangia contain from I to 8 megaspores. 

 The microsporophyll bears microsporangia (pollen sacs) which 

 contain microspores (pollen grains). The female gametophyte in 

 the Spermophytes is still more limited in its development than 

 even in the highest Pteridophytes (as Selaginella and Isoetes) 

 and remains wholly within the megaspore or embryo-sac. As a 

 result of fertilization of the egg-cell an embryo is produced which 

 consists of root, stem, and one or more cotyledons and which with 

 the integuments covering it constitutes the seed. 



Spermophytes embrace two well-defined groups, namely, (i) 

 Gymnosperms, or naked-seeded plants, and (2) Angiosperms, or 

 enclosed-seeded plants. 



GYMNOSPERMS. 



In the Gymnosperms the ovules, each of which contains a 

 megasporangium (nucellus), are borne on an open sporophyll 

 (carpel), and thus are exposed, as are also the seeds developed 

 from them. In the Angiosperms the ovules are borne within 

 closed sporophylls, and are thus protected or covered until the 

 seeds, which develop from them, mature. 



The Gymnosperms represent an ancient group of plants and 

 were more numerous during the Triassic period than now. They 

 are mostly shrubs and trees, and do not shed their leaves period- 

 ically as the Angiosperms do, and hence are known as " ever- 

 greens." As in some of the Pteridophytes (Lycopodium, Equi- 

 setum), the sporophylls occur in groups forming cones or strobiles 

 (Fig. 72). They not only differ in external appearance from the 



