1454 A TEXTBOOK OF THEORETICAL BOTANY 



though the existence of such abortive divisions in the cytoplasm might indi- 

 cate that the cellular type is the more primitive. 



The number of successive divisions which take place depends on the 

 size of the embryo sac, but as this is rapidly expanding at this time, the num- 

 ber may be considerable and in some cases several hundreds or even thou- 

 sands {Malva, Mains) of free nuclei may be formed. On the other hand, in 

 small sacs, wall formation may begin when there are no more than from four to 

 sixteen nuclei. 



The expansion of the embryo sac is accompanied by the enlargement of a 

 central vacuole, which presses the cytoplasm out against the periphery, 

 and in this thin layer of cytoplasm the multiplying nuclei are spaced out, 

 roughly equidistant from each other. There may, however, be closer aggre- 

 gation at the two poles, around the embryo or the suspensor, and at the anti- 

 podal end respectively. The latter nuclei may also become larger than the 

 others. In some isolated cases, e.g., Miisa and Hypericum, some of the anti- 

 podal endosperm nuclei may be separated from the others by an enclosing 

 membrane forming a multi-nucleate cyst (see Fig. 1301), which in Hyperi- 

 cum sends down a haustorial extension into the chalaza. Swamy has called 

 this the Hypericum type of endosperm. 



Wall formation generally begins at the periphery of the sac, cell plates 

 growing inwards centripetally. How these originate is not altogether clear. 

 They may start as granular deposits or as layers of minute vacuoles, and in 

 many plants these are laid down on secondarily formed spindles. In a few 

 cases, e.g., Asclepias, they seem to start by the fission of the cytoplasm along 

 certain lines. The areas between these walls are at first incompletely de- 

 limited, but later inner walls appear which accomplish the formation of a 

 peripheral layer of cells. 



Hegelmaier has distinguished the following varieties of wall formation 

 in the endosperm. 



1. A peripheral layer of cells is completed, as above described. These 

 cells divide periclinally until the whole of the interior space is filled. A 

 widely spread type. 



2. A peripheral layer of centripetal walls appears and these prolong 

 themselves until they meet in the middle of the sac, when division into cells 

 follows (compare the process of " alveolation " in Gymnosperms). Occurs 

 in some Cucurbitaceae, Bocconia, Scabiosa and Euphorbia. 



3. Wall formation starts from the micropylar end and the chalazal end 

 may remain undivided. Occurs in many plants, e.g., Cytisus, Polygonum, 

 Rumex, etc. 



4. Endogenous type. The embryo sac is filled with cytoplasm and wall 

 formation occurs simultaneously throughout. A rare type, known definitely 

 in Eranthis and perhaps in Tricyrtis, Vincetoxicum and one or two other 

 plants. 



These types are useful descriptively but they do not cover the large 

 number of variant and intermediate cases which are known. 



The cells which are first formed may include several nuclei (Fig. 1334) 



