GROUPS OF PLANTS. 71 



furnish a large part of the food of man and other animals, as well 

 as materials for clothing, shelter, fuel and divers other purposes. 

 In this group of plants there is the highest differentiation of tis- 

 sues and the most complicated structure. The one character 

 which especially distinguishes them from the lower groups of 

 plants is that of the production of seeds. 



The plants have for the most part well differentiated stems 

 and leaves, and represent the sj^orophyte 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. 49, 50, 56, 85). Each megasporangium 

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

 Selaginella the megasporangia contain from i to 8 mega- 

 spores. The microsporophyll bears microsporangia (pollen 

 sacs) which contain microspores (pollen grains). The fe- 

 male gametophyte in the Spermophytes is still more limited in 

 its development than even in the highest Pteridophytes (as Sela- 

 ginella 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 mega- 

 sporangium (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 sporo- 

 phylls, and are thus protected or covered until the seeds, which 

 develop from them, mature. 



