346 BOTANY: PRINCIPLES AND PROBLEMS 



the spores are not uniform but that microspores and megaspores, 

 borne in separate sporangia and on separate sporophylls, germi- 

 nate into male and female gametophytes, respectively. The 

 seed plants are likewise heterosporous. As in the lower groups, 

 they develop microspores (now called pollen grains). These 

 are borne in a microsporangium (now called an anther), arising 

 from a microsporophyll (now called a stamen). No very radical 

 change is evident here, but in the case of the female structures 

 we find some marked innovations. 



The Ovule and Its Contents (Figs. 217 and 229).— The mega- 

 sporangium, now known as the nucellus, produces only one 

 functioning megaspore, for the other three members of the tetrad 

 which begin to develop soon disappear. Furthermore, the 

 sporangium does not burst and liberate this spore but retains it, 

 instead, and allows it to germinate and produce the female 

 gametophyte within the tissues of the sporangium (nucellus), 

 nourished by the parent sporophyte. The female gametophyte 

 (now called the embryo-sac) is a small, roundish group of cells 

 filled with" food and bearing at one end one or more archegonia 

 or structures comparable to them. The whole is embedded in 

 the tissue of the nucellus and is never freely exposed. Among 

 the highest forms it suffers such reduction that resemblance to a 

 gametophyte becomes very faint. The nucellus and its enclosed 

 embryo-sac are completely surrounded and protected by one 

 or two coats or integuments except for a small opening, the 

 micropyle, which occurs just opposite the point where the arche- 

 gonia are borne in the embryo-sac beneath. The whole struc- 

 ture — integument, nucellus, and embryo-sac — is known as the 

 ovule, and after fertilization, for which it is now prepared, the 

 ovule will develop into a seed. It is closely attached to 

 the megasporophyll, which is now called a carpel. 



Pollen (Fig. 218). — The microspore or pollen grain, produced 

 in the microsporangium, in the mean time germinates and 

 produces within itself the male gametophyte. This is greatly 

 reduced and consists at most of a very few cells, in the higher 

 forms of only two — the generative cell and the tube-nucleus. 

 These are all that remains of the male sexual generation. Germi- 

 nation of the microspore usually takes place, at least in part, 

 before it is liberated by the breaking open of the anther wall. 

 After this event, the pollen grain is transferred, by wind, insects 

 or other means, either directly to the ovule or to a receptive 



