CHAPTER VI 



CYCADALES— Continued 



The reduction of chromosomes, in both megaspore mother-cells 

 and in microspore mother-cells, brings back the original x number, 

 which is the only number in plants below the level of sexuality. Since 

 the terms gamctophyte and sporophyte are coincident with x and 2x 

 generations, in plants from the Hepaticae through the angiosperms, 



and in many Thallophytes, stu- 

 dents arc likely to think the terms 

 are synonymous. It is customary 

 to call the x generation the gamcto- 

 phyte because, in the higher plants, 

 it produces only gametes; and the 

 2x generation, the sporophyte, be- 

 cause, in the higher plants, it is the 

 only generation producing spores. 



Fig. 114. — Zamia floridana: En- 

 larging megaspore; X930. — After Dr 

 F. Grace Smith.s^ 



THE FEMALE GAMETOPHYTE 



The megaspore is the first cell 

 of the female gametophyte. It en- 

 larges considerably, absorbing some 

 of the neighboring cells before it 

 divides (fig. 114). By the time the first division of the nucleus has 

 been completed, there is a great change in the surrounding cells, 

 which form a layer, quite common in gymnosperms, called "spongy 

 tissue" by Strasburger (fig. 115). The earlier free nuclear divisions 

 are simultaneous, and the nuclei are crowded outward by a large cen- 

 tral vacuole, so that most of the protoplasm, containing the free 

 nuclei, is in a thin peripheral layer (fig. 116). Free nuclear division, 

 in Dioon edule, continues until there are about 1,000 free nuclei, be- 

 fore walls begin to form. 



Cell walls appear first at the periphery, and wall formation pro- 

 ceeds toward the center until the entire gametophyte becomes cellu- 



uS 



