PHYSIOLOGY OF THE RED MANGROVE. 623 



than those of the endosperm and that the union between the two 

 layers of cells is very close. Cook further has divided the periods 

 of growth of the embryo into three definite periods ; first, the first 

 growth of the cotyledons, during which they enlarge and are the 

 means of storing up the food for the later growth ; second, the 

 cotyledons almost cease growing, w^hile the hypocotyl elongates and 

 the plumule is forming, and the beginnings of vascular elements 

 take shape ; third, the second growth of the cotyledonary body 

 which pushed out the region of union of the cotyledons and the 

 hypocotyl so that the cotyledonary body projects like a green collar 

 beyond the apex of the fruit. An absciss layer is then formed at 

 the base of the plumule and the hypocotyl drops off. 



POLYEMBRYONY. 



The presence of four ovules in the young condition of the fruit 

 and the habitual development of only one of these into a seed 

 naturally leads the investigator to look for polyembryony in the 

 genus. This condition actually does happen at rare intervals and 

 has been noted by a few observers. Warming quotes Piso,®- who 

 figures this rare phenomenon of two or more radicles pushing out 

 from one fruit. Baron Eggers^^ is also quoted as estimating from 

 his observations on this species in the West Indies that polyem- 

 bryony occurs three times in one thousand cases and Du Petit 

 Thouars^* is also reported to have observed this. Polyembryony 

 may Qccur according to the wade usage of the term by some botan- 

 ists, /. e., two or more embryos may develop wnthin one embryo sac 

 by the formation of several embryos, one of which originates from 

 the egg and it is this which Warming figures in PI. VII.-VIII., or 

 in the wider sense of two or more ovules germinating from one 

 fruit. The dilTerence may easily be seen on cutting away the fruit 

 w^all; if only one seed is present, it can only be interpreted as true 

 polyembryony. In the second case two or more seeds would be 

 noticed. 



•'- Piso, G., loc. cit. 

 ^3 Eggers, H., loc. cit., p. i8o. 



^■i Thouars, Albert du Petit, " Notice sur le Manglier," Desvaux's Journal 

 dc Botanique, t. 3, p. 27, 1813. 



PROC. .A.MER. PHIL. SOC, VOL. LVI, GO, J.\XU.\RY 8, I918. 



