HAUSTORIAL ORGANS OF SEEDLINGS 



247 



absorbing organs. Structures of this nature, comparable to haustoria 

 and often actually designated by this name, are especially characteristic 

 of Monocotyledonous seedlings. Most often it is the distal portion of 

 the cotyledon that remains embedded in the endosperm during germi- 

 nation in order to serve as an organ of absorption. A familiar instance 

 is that of the Date Palm. 



The absorbing tissues of germinating embryos resemble those of 

 roots in the fact that they can be referred to two distinct types, of 

 which one is more specialised than the 

 other. The simpler type is charac- 

 terised by the possession of absorbing- 

 cells with flat, or at most slightly 

 papillose, outer walls. The absorbing- 

 surface exposed is thus relatively 

 small, and the absorption of reserve- 

 materials proceeds but slowly, weeks 

 or even months elapsing before the 

 endosperm is completely exhausted. 

 This slow absorption is, however, 

 necessitated by ecological conditions 

 in the case of many plants, which 

 accordingly find the simple form of 

 absorbing tissue quite sufficient for C 

 their needs ; the latter is, indeed, the 

 only type that is known to occur 

 among Palms, and in the Liliaceae, 

 Iridaceae, Zingibeeaceae, Mabanta- 

 ceae, Cyperaceae, etc. 



The embryos of Grasses are fur- 

 nished with a more elaborate form of 

 absorbing tissue, 127 which is located 

 on the dorsal surface of the scu- 



tellum, where the latter abuts against the endosperm (Fig. 95 a). 

 Even in the resting condition of the embryo, the absorbing cells are 

 elongated at right angles to the surface of the scutellum [forming the 

 so-called scutellar epithelium] ; at this stage they are in uninterrupted 

 contact with one another laterally. During germination the scutellum 

 increases considerably in size ; since the epithelial cells do not expand 

 laterally in the same degree, they become partially or completely 

 separated from one another. In this way numerous intercellular spaces 

 open to the outside [i.e. towards the endosperm] are produced, while 

 individual cells, or small groups of cells, become isolated all round (Fig. 

 95 b). The increased surface which becomes available in this way is 



B 



Fig. 95. 



A. 



Embryo of Wheat in the resting 

 condition (L.S.): s, s, scutellum (its absorbing 



epithelium is shaded), xlii. B. Part of the 

 scutellar epithelium at an advanced stage 

 of germination, showing the tubular absorb- 

 ing cells. x230. V. Part of the scutellar 

 epithelium at an advanced stage of germina- 

 tion ; absorbing cells in T.S. D. Cells of the 

 scutellar epithelium from a seedling of Stipa 

 Calamagrostis. x 320. 



