244 ABSORBING SYSTEM 



possibilities the three first-mentioned may be selected for further 

 illustration. 126 



In several species of Linum the lower region of the embryo-sac (or 

 megaspore) serves as a haustorium ; this portion, which ultimately 

 becomes marked off from the upper part by a constriction, contains 

 protoplasm and receives a few of the endosperm-nuclei, but no cell- 

 walls are ever formed between the latter. In such a case as this the 

 haustorial portion of the embryo-sac remains unbranched. In other 

 instances an increased exposure of surface is effected by the more or less 

 extensive branching of the haustorium. This more advanced condition 

 is exemplified, according to Billings, by Globularia cordifolia. Here 

 the upper extremity of the embryo-sac grows out as a tubular process 

 through the micropyle, spreading all over the upper end of the ovule 

 and coming into contact with the wall of the ovary ; this process puts 

 forth a number of filamentous outgrowths, some of which push their way 

 downwards between the growing seed and the pericarp, while others 

 extend upwards along the funicle towards the placenta. 



The formation of endospermic haustoria was first observed by Treub 

 in Avicennia officinalis (Verbenaceae). In this plant the endosperm 

 together with the enclosed embryo passes bodily through the micropyle 

 and into the cavity of the ovary, with the exception of a single huge 

 and abundantly branched cell (the " cellule cotyloide "), which acts as a 

 haustorial organ ; the branches of this haustorial cell ramify throughout 

 the nucellus, and finally even invade the placenta. In the above- 

 mentioned Globularia cordifolia the lowermost cells of the endosperm 

 grow out in tubular fashion into the adjoining integument ; the latter 

 thus becomes riddled by a system of haustorial tubes, which ultimately 

 absorb the whole of its substance. 



The author has elsewhere fully described the very remarkable 

 endospermic haustoria discovered by him in certain viviparous Man- 

 groves. In Bruguiera eriopetala (Bhizophoraceae) the endosperm, 

 which forms a thin layer surrounding the four basally connate 

 cotyledons, grows out into numerous haustorial lobes resembling root- 

 hairs. These arise in the following manner. During the early stages 

 of development of the fruit, the primary endosperm is entirely 

 obliterated by the growing cotyledons, except for isolated approximately 

 hemispherical cells with abundant protoplasmic contents, which adhere 

 closely to the cotyledonary surface. These persistent endospermic 

 cells later develop into multicellular discs or " islands " of endosperm, 

 which ultimately become more or less fused with one another. This 

 peculiar endospermic tissue puts forth numerous one- to many-celled 

 haustorial processes, which penetrate the very loose parenchyma 

 of the integument (Fig. 94 b). It is noteworthy that, at a number 



