SUBKINGDOM SPERMATOPHYTA 



329 



showing a rudimentary annulus like that of Angiopteris. The spores are bilat- 

 eral in form, and the pollen-sac opens by a longitudinal cleft. 



The macrosporangia in Cycas are borne laterally upon very slightly modified 

 sporophylls (Fig. 290, A). The ovule consists of a central part, the nucellus, 

 enclosed by a thick integument, which becomes pulpy and bright colored, look- 

 ing like a large cherry or plum. 



The young ovule develops a mass of sporogenous tissue, a single cell of which, 

 after preliminary division, gives rise to the single macrospore or Embryo-sac. 

 This develops a double wall, like the macrospore of the Pteridophytes, but never 

 escapes from the sporangium. 



The Gametophyte 



The microspore, before it escapes from the pollen-sac, has already divided 

 into three cells (Fig. 290, E), one of which is much larger than the others. Of 

 the two smaller cells the inner one is 

 the antheridial cell, and is carried 

 into the pollen-tube when the spore 

 germinates. 



The development of the female 

 gametophyte (Figs. 291, 292) is much 

 like that of Isoetes. The primary 

 nucleus of the macrospore divides 

 into a large number of free nuclei, 

 between which the primary cell-walls 

 arise simultaneously. Finally the 

 spore becomes filled with the pro- 

 thallial tissue, and several archegonia 

 are developed at the apex. These 

 have each a very large egg-cell, from 

 which a canal-cell is later cut off. 

 Two neck-cells are developed. 



Fertilization 



The fertilization (Fig. 291) has FIG. 291. A, Cycas revoluta, pollen-tube 



been recently studied in Zamia inte- 

 grifolia and Cycas revoluta. At the 

 apex of the ovule there is developed 

 a cavity, the pollen-chamber, into 

 which the pollen falls, and begins its 

 germination. The pollen-tube grows 

 into the tissue of the nucellus, from 

 which it doubtless obtains its nourish- 

 ment. The upper end, to which the 



containing the large antheridial cell, an ; 

 the nucleus has not yet divided, but two 

 blepharoplasts, 6, are present ; pg, wall of 

 pollen-grain. B, C, Zamia integrifolia. 

 B, pollen-tube containing the two large 

 spermatozoids (X 75). C, upper part of 

 ovule (somewhat diagrammatic), show- 

 ing three germinating pollen-spores in the 

 pollen-chamber above the archegonia, ar. 

 (A, after IKENO. B, C, after WEBBER.) 



membrane of the pollen-spore is still 

 attached, increases very much in size, and becomes distended with the watery 

 fluid within. Shortly before fertilization is to take place the nucleus of the 

 antheridial cell divides, and two very large spermatozoids, each provided with 

 a spiral ciliated band, derived from the large blepharoplast, are set free within 

 the tube. The pollen-tube then bursts, and discharges the fluid contents, 

 together with the spermatozoids, into the chamber above the archegonia, into 

 which the spermatozoids then enter as they do in the Ferns. 



