THE ENDOSPERM 241 



each. In subsequent stages only the four central cells divide further 

 and give rise to the body of the endosperm, while the terminal cells 

 remain undivided and take up a haustorial function. 



An interesting feature, described in special detail in Utricularia 

 vulgaris americana (Wylie and Yocom. 1923), is that, because of the 

 disappearance of the nucellar epidermis and the protrusion of the 

 embryo sac through the micropyle, the micropylar haustorium comes 

 to lie in direct contact with the nutritive tissue of the placenta (Fig. 

 134D, E). The walls of the placental cells frequently break down, 

 so that their nuclei become scattered in a common mass of cyto- 

 plasm along with the nuclei of the haustorium. The placental nuclei 

 enlarge, and some of them fuse to form "tuber-like" structures 

 many times their normal size. The nuclei of the haustorium, which 

 are usually still larger and more chromatic, become lobed and bud 

 out into separate masses. "These with the placental nuclei lying 

 in the fluids of the haustorium offer a most peculiar assemblage of 

 nuclear structures" (Fig. 134F). The chalazal haustorium, although 

 less massive, also digests its way through the intervening cells and 

 ultimately comes in contact with the cells of the epidermis, which 

 become perceptibly weakened at this point. 



The occurrence of endosperm haustoria, both micropylar and 

 chalazal, is also a universal feature of the Scrophulariaceae. A 

 widely distributed mode of development is illustrated by Lathraea 

 (Glisic, 1932) (Fig. 135). Of the two primary endosperm chambers, 

 the chalazal functions directly as a haustorium after undergoing one 

 nuclear division. The micropylar divides vertically and then trans- 

 versely to give rise to two tiers of two cells each. Of these, the 

 uppermost tier gives rise to two binucleate haustorial cells which 

 later fuse to form a single tetranucleate structure, and the lower 

 gives rise to the endosperm proper. 



There are several variations of this scheme, which have been 

 described by Krishna Iyengar (1937, 1939, 1940, 1941, 1942) in recent 

 publications. Although the chalazal haustorium usually comprises 

 a single binucleate cell, in certain plants like Celsia, Isoplexis, and 

 Verbascum it is tetranucleate, while Vandellia, Sopubia, and Alonsoa 

 (Fig. 136D) have two uninucleate prongs which may later fuse to 

 form a single binucleate cell. The micropylar haustorium is differ- 

 entiated somewhat later, but it is more aggressive and persists for 

 a longer time. In some plants there are four uninucleate cells, 

 which may remain as such as in Alonsoa (Fig. 136), Isoplexis, 



