138 MORPHOLOGY OF GYMNOSPERMS 



(fig. 151). The number of archegonia varies considerably: in 

 Cycas revoluta there are two to eight, with three the most fre- 

 quent number; and in Dioon edule, one 



-ffy- tlTS 7V; *^° ^^^' ^^'•^ ^°^^ ^^ raost frequent. All 



^ Mk W the cycads that have been examined fall 



/ ^ within these limits, with the single ex- 



FiG. iK,i.— Dioon edule: ception of Microcycas. In this genus 



development of the archegonial ^ / \ i a-l 



, , , ,, ,.f Caldwell (■;2) reports more than 200 



chamber; a, the condition in \j / jr 



December, the endosperm some- archegonia scattered over the entire sur- 



what elevated over the arche- face of the gametophyte, and some of 



gonia; /. in February the ^^^ ^^^^ -^^^ ^^ median 



chamber appearing as a slight 



depression; a, in April, shortly cleft where the centripetal growth of the 

 before fertilization; all natural gametophyte has not Completely replaced 

 size. - After Chamberlain ^he large central vacuole. Usually the 



archegonia are grouped at the micropylar 

 end of the gametophyte. 



In the heterosporous pteridophytes the megaspore wall is thick 

 and highly differentiated, and it has long been known that the endo- 

 sperm of Cycas is surrounded by a membrane which is the homologue 

 of the spore coat of the pteridophytes. With the retention of the 

 megaspore in the sporangium, the spore coat would naturally become 

 reduced as the retention became more and more pronounced, so that 

 the thickness of the megaspore coat, or megaspore membrane as it 

 is more usually called, is one of the factors to be considered in esti- 

 mating the relative antiquity of the different forms. At present, 

 however, little dependence can be placed upon this factor, because the 

 thickness of the membrane differs decidedly at different stages in the 

 development of the ovule. In Dioon edule (46) the thickness of the 

 membrane during free nuclear division in the endosperm is about 3 /*, 

 just before the beginning of the archegonial chamber about 5 /*, while 

 in the germinating seed it has increased to 9-10 /".. Not enough stages 

 have been measured in any other form to make comparisons reliable. 



The most extended investigation of this subject is that of Thomson 

 (45), who studied not only cycads, but also the other living groups 

 of gymnosperms. He finds that the membrane is differentiated 

 into two layers, an exospore, which is suberized, and an endospore 

 of complex composition. The inner region of the endospore, next 



