THE ENDOSPERM 303 



considered without the compHcation of "double fertilization," which 

 enters the picture only after the sac is mature. 



THE ENDOSPERM 



The fusion of the second male nucleus with the two polar nuclei 

 forms the primary endosperm nucleus, which typically divides to form 

 tissue that serves to nourish the developing embryo and often, later, 

 the germinating seed. This second fertilization was unknown until 1898, 

 but, after its discovery, was soon found in many taxa and came to be 

 accepted as a basic step in the development of the embryo. When first 

 known, it was called "vegetative fertilization" and considered a stimula- 

 tion in the development of nutritive tissue for the embryo. There have 

 been reports that, in some taxa, endosperm may develop from one or 

 both polar nuclei, without fusion with a male nucleus, but these taxa 

 should be reinvestigated. In embryo sacs where supernumerary nuclei 

 unite in the formation of the primary endosperm nucleus or where 

 some of the nuclei are already triploid, as in the Fritillaria and Plum- 

 bagella types, the fusion nucleus and the tissue formed by it are 

 polyploid. 



Types of Endosperm Formation 



Differences in the method of formation of early stages of the endo- 

 sperm form the basis for a distinction of types. Where the first divisions 

 are free nuclear (Figs. 113B and 115) — without immediate wall forma- 

 tion about the new cells — the endosperm is termed nuclear (Fig. 115). 

 The number of free nuclear divisions ranges from one to several, and 

 the free-nucleate tissue so formed may persist as such until destroyed 

 by the embryo, or may become walled. In sacs where walls are formed 

 after the first divisions and may be formed at some or all of the later 

 divisions, the endosperm is celhilor (Fig. 116). No clear line exists 

 between these types. A combination of the two types — the first division 

 dividing the embryo sac transversely into two parts, one of which re- 

 mains free nuclear and the other with walled cells — is called the 

 helobial type, because it occurs frequently in the Helobiales. Probably, 

 no endosperm is formed in the greatly reduced and peculiar embryo 

 sac of the Podostemonaceae; and, in the Orchidaceae, the endosperm is 

 small in amount and ephemeral. There seems to be no evidence as to 

 which of the two types — nuclear or cellular — is the more primitive. 



Lobes of the sac containing free nuclei may develop into extensive 

 haustoria, which provide nourishment for the rest of the sac and the 

 embryo. And there are many other modifications of form and structure 

 in the sac when the endosperm is developing. 



