19 1 q] SCHERTZ—SCROPHULARIA 447 



greatly thicken their walls, and their contents consist now of 

 crystalloid protein materials (fig. 36) along with other stored 

 foods. 



When the endosperm is formed, it first occupies only the space 

 outlined by the early stage of the embryo sac (fig. 22). As the 

 development of the endosperm proceeds, enzymotic processes set 

 in and the tapetal cells, along with several layers of cells just 

 without, are absorbed and changed into endosperm structure. 

 When the endosperm consists of only two layers of cells throughout 

 the length of the embryo sac (figs. 29, 30, 32), several layers of cells 

 are evident in the surrounding tissues. At a later stage (fig. 33) 

 8-10 layers of cells were observed in the cross-section of the endo- 

 sperm, and then not so many layers (3-6) of cells were found in the 

 surrounding tissues. It is evident that the endosperm tissue is 

 being increased at the expense of the surrounding layers of cells. 

 The egg up to this time has not divided. When the egg divides, 

 the endosperm (fig. 26) in cross-section is many layers thick, while 

 the remaining cells of the surrounding tissues have collapsed and 

 become compressed into a thin layer. The cells at the end of the 

 embryo sac are not so greatly changed as are the cells surrounding 

 the sides, and even when the embryo is mature, the cells at the ends 

 of the sac are still noticeable but are slightly compressed. At about 

 the time of the quadrant stage of the embryo (fig. 28), enzymes are 

 secreted and the endosperm tissue surrounding the embryo, extend- 

 ing greatly toward the chalazal end, becomes disintegrated. At 

 this stage the embryo grows rapidly, and soon the cotyledons differ- 

 entiate. The embryo is now found lying within the endosperm. 



The cells of the endosperm are of two types. Those which imme- 

 diately surround the embryo are long and narrow and mostly devoid 

 of all protein matter, their relation to the embryo evidently leading 

 to a loss of all their cell contents. The cells surrounding these have 

 greatly thickened cell walls and are gorged with protein crystals 

 (fig. 36) and other stored foods. The endosperm is entirely sur- 

 rounded by a thick hardened coat which stains heavily with Dela- 

 field's haematoxylin, and from all appearances serves to protect the 

 seed from loss of moisture. This coat is the testa. Remnants of 

 the single integument persist as an outer covering composed of two 

 or more layers of cells. 



