HAUSTORIAL ORGANS OF EMBRYOS 



245 



of points, individual cells of the endosperm give rise to tubular 

 processes on their inner side, which insert themselves between 

 the radially elongated palisade-like epithelial cells of the cotyledons 

 (Fig. 94 a). These latter processes are of course not actually haus- 

 torial organs, but evidently serve to produce the closest possible 

 connection between the absorbing endosperm and the seedling which is 

 destined to receive the absorbed material. 

 Here the principle of maximum exposure 

 of surface affects the structure, not only 

 of the receptive part of the endosperm, 

 but also of its discharging surface. 



In Aegiceras majus (Myrsinaceae), 

 which is likewise viviparous, endospermic 

 haustoria occur in the form of wedge- 

 shaped lobes and folds or villi, which are 

 not infrequently branched ; but these 

 developments are strictly confined to the 

 placental region, where the influx of 

 plastic and reserve-materials takes place. 

 That the presence of these endospermic 

 haustoria must facilitate the nutrition of 

 the large seedlings of viviparous plants 

 is sufficiently obvious. One is involun- 

 tarily reminded of the richly branched 

 chorionic villi and lobes of the Mam- 

 malian placenta, which are themselves 

 indeed nothing more than elaborate 

 haustorial developments. 



Where haustoria are formed by the 

 embryo itself, they generally originate 

 from the suspensor ; here again the 

 dominating influence of the principle of 

 maximum exposure of surface is often 

 beautifully illustrated. Thus Treub has 

 described how, in species of Phalaenopsis (P. yrandijiora, P. Schil- 

 leriana, etc.), each of the four cells of the suspensor grows out into 

 a pair of long tubular structures ; one set of these haustorial tubes 

 penetrates the micropyle, while the rest surround the embryo like a 

 bundle of Fungus-hyphae. In another Orchid, Stanhopea ocu/afa, the 

 fertilised egg-cell first of all divides to form a ten- to fifteen-celled 

 spherical structure, called the proembryo ; a single cell of this pro- 

 embryo gives rise to the true embryo, while all the rest grow out into 

 long tubes, which push their way among the cells of the nucellus right 



Fig. 94. 



Endospermic haustoria of Bruguiera 

 eriopetala. A. Portion of the integu- 

 ment with the adhering palisade-like 

 epithelium of a cotyledon ; the latter is 

 traversed by several small branches of 

 the endospermic haustoria. B. One of 

 the larger haustoria; the round celled 

 tissue belongs to the integument, the 

 palisade-like layers to one of the coty- 

 ledons. 



