448 



SPERMATOPHYTES (SEED PLANTS) 



when they are short and fleshy, they fit together like the kernels 

 on an ear of Corn. The ovules are borne separately near the 

 base of the megasporophyll and as shown in Figure 398. In some 

 Cycads each megasporophyll bears only two ovules, while in 

 others, as Figure 398 shows, a larger number may be present. In 

 some Cycads the megasporophylls are much branched like foliage 

 leaves, and the sporangia appear to be transformed lower branches 

 or pinnae. Megasporophylls of this type suggest the relation- 

 ship of sporophylls to foliage leaves. 



The young megasporangium or ovule contains four megaspores, 

 which are enclosed by two distinct coverings of sterile tissue. 

 The inner covering is the nucellus, which surrounds and encloses 



FIG. 398. Ovulate strobilus and megasporophylls in Cycads. At the 

 left, an ovulate strobilus; at the right, two types of megasporophylls, show- 

 ing the ovules (o). 



the megaspores, and the outer one is the integument, which grows 

 up from the base of the ovule and forms a covering over the 

 nucellus. The integument is a protection for the nucellus, and, 

 when the ovule develops into a seed, it is transformed into a seed 

 coat. At the outer end of the megasporangium where the integu- 

 ment closes over the nucellus, a small opening or micropyle is 

 left which leads into a cavity, called the pollen chamber, into 

 which a beak-like portion of the nucellus projects (Fig. 399). 



Female Gametophyte. Only one of the four megaspores in 

 the megasporangium develops. The other three disappear and 

 all of the space and food is therefore given over to the develop- 

 ment of one gametophyte. The megaspore germinates in the 



