252 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



free nuclear divisions, resulting in a number of nuclei distributed in 

 the peripheral layer of cytoplasm. Eventually cell formation occurs 

 here also and it becomes virtually impossible to distinguish the two 

 portions of the endosperm, one derived from the micropylar and the 

 other from the chalazal chamber. 



Relationships between Different Types of Endosperm. We are 

 still in the dark regarding the phylogenetic sequence of the evolution 

 of the various types of endosperm. There are no doubt transitional 

 forms which interconnect the Nuclear, Helobial, and Cellular types 

 so that the classification is one of convenience rather than of abso- 

 lute accuracy, but whether the series is to be read from the Nuclear 

 toward the Cellular type or vice versa is not clear. 



In a discussion of some of the transitional forms, reference may 

 first be made to Hypericum (Stenar, 1938). The initial divisions 

 of the primary endosperm nucleus are free nuclear (Fig. 143 A-C), 

 but at the eight-nucleate stage the two uppermost nuclei come to 

 lie in a dense mass of cytoplasm adjacent to the zygote (Fig. 143D). 

 Following the next division, four nuclei are seen in the micropylar 

 part of the embryo sac, 11 are distributed in the peripheral layer of 

 cytoplasm, and one becomes delimited in a dense mass of cytoplasm 

 at the chalazal end (Fig. 14SE). With further divisions, the endo- 

 sperm may be said to comprise three more or less distinct regions — 

 micropylar, chalazal, and central. The first two have a denser 

 cytoplasm and seem to be separated from the third, which has a 

 large central vacuole, by thin plasma membranes (Fig. 143F-7). 

 At about the octant stage of the embryo, the distinction between 

 the micropylar and the central portions becomes less sharp, and 

 eventually the two merge into one another. The chalazal portion, 

 however, forms a coenocytic "cyst," whose upper surface "seems to 

 become hardened and delimited from the rest of the embryo sac" 

 (Swamy, 19466). At this stage the endosperm of Hypericum may, 

 therefore, be mistaken for one of the Helobial type (Palm, 1922), 

 although the developmental studies made by Stenar and Swamy 

 leave no doubt that it is Nuclear. It does illustrate, however, a 

 kind of transition between the Nuclear and the Helobial types. 



Lappula echinata, a member of the Boraginaceae, also shows a 

 condition which may be regarded as intermediate between the 

 Nuclear and the Cellular type. As reported by Svensson (1923) 

 the first division of the primary endosperm nucleus is followed by 



