CONIFERALES 325 



or two of a second tetrad. Dupler'^^ found several ovules in which 

 three female gametophytes had reached the archegonium stage, with 

 a couple more in the free nuclear stage. In Taxus haccata several in- 

 vestigators have failed to find more than one megaspore germinat- 

 ing. Dupler's findings are conclusive for T. canadensis. More than 

 one megaspore has been observed to develop in Sequoia, ^'^'^ Cunning- 

 hamia,'^°^ S ci ado pity s,^'^^ Taxodium,^^^ and Cryptomeria.^'^^ More than 

 one is rare in Pinus.^^^ 



In the early stage of the megaspore, no megaspore membrane, 

 distinct from the ordinary cellulose wall, can be distinguished; but 

 as the free nuclear stage advances, and the cellular stage begins, a 

 genuine spore coat appears, and may reach considerable thickness. 

 The thickness of some megaspore membranes, as given by Thom- 

 SON,^33 are ^g follows: Biota orientalis, 1.7 microns; Sequoia sem- 

 pervirens, 2.7 microns; Pinus sylvestris, 3 microns; P. resinosa, 4.2 

 microns; Abies halsamea, 4.6 microns; Larix americana, 4.7 microns. 

 These measurements indicate the range of thickness in the order. 



The chemical composition of the membrane is very complex. On 

 the outside it is suberized, and, on the inside, chemical tests indicate 

 a substance related to pectin. Between the two, there is cellulose 

 which gradually changes into the suberin of the outside^ 



Thomson^" describes two layers, an exosporium and an endo- 

 sporium, and there is no doubt that stained material, which is easily 

 photographed, seems to support his view. However, three layers 

 can be photographed in Dioon edule, where it is doubtful whether 

 there is more than one. It is more than doubtful whether there are 

 two layers originating Hke the exine and intine of microspores. 



In spores which are to be shed, spore coats are highly developed, 

 and the megaspore of gymnosperms still retains a highly developed 

 spore coat. When the megaspore began to be retained in the mega- 

 sporangium (ovule) there was no longer need for a protective coat, 

 and it became more and more reduced, still appearing in the coni- 

 fers, but not so thick in living forms as in the carboniferous gymno- 

 sperms. In the Gnetales^" the coat is very thin — 1.3 microns in 

 Welwitschia—Sind in the angiosperms it has disappeared as a recog- 

 nizable spore coat. 



During its early development, the female gametophyte is sur- 



