500 



SECRETORY AND EXCRETORY SYSTEMS 



somewhat oily protoplasmic matrix and a centrally situated nucleus. 

 In shape these cells are prismatic and often somewhat elongated radially 

 (Secale, Triticum, Arena). Typically the aleurone-layer is only one 

 cell thick, but in Oryza sativa, Arrhenatherum < latum and certain other 

 species it becomes two-layered, and in Hordeum even three- to four- 

 layered, owing to tangential division. The inner and lateral walls are 

 more or less thickened, and have been shown by Tangl to be traversed 

 by innumerable delicate protoplasmic strands. 



When germination is in progress, the aleurone-layer presents a very 

 different appearance. While the starch-containing portion of the 

 endosperm becomes converted (in Rye, Wheat, Oat, etc.) into a soft 



pasty mass, the aleurone-layer remains 

 perfectly continuous, although it be- 

 comes completely separated from the 

 adjoining cells of the starchy endo- 

 sperm. The reserve-materials stored 

 in the aleurone-cells are utilised in 

 f the development of massive proto- 

 plasts, such as are characteristic of 

 glandular cells in general (Fig. 204). 

 The fact that these reserve-materials 

 of the aleurone-layer are utilised 

 locally in the manner described, 

 instead of being translocated to a 

 distance, clearly proves that the 

 layer in question cannot form part of the storage system, but must 

 be concerned with some special function demanding the presence 

 of massive protoplasts. Even in the later stages of its existence the 

 aleurone-layer never seems to give up any plastic materials to the 

 embryo. On the contrary, the protoplasts in this layer appear to 

 undergo fatty degeneration before they die, the cell-cavities becoming 

 filled with an increasing number of highly refractive globules, some of 

 which, at any rate, exhibit the reactions characteristic of fatty oils. 

 These globules are still to be found in the dead aleurone-cells, long 

 after the starchy endosperm has been entirely depleted, and the seedling 

 has attained to a state of nutritive independence. 



That the above-described aleurone-cells, with their dense proto- 

 plasmic contents, do actually secrete diastase during germination, is 

 rendered highly probable by a fact which was known to Tangl among 

 others, namely, that corrosion and solution of starch-grains begin much 

 earlier in those endosperm cells that lie immediately beneath the 

 aleurone-layer, than they do in the central portion of the endosperm. 

 The aleurone-layer of Rye is imperfectly developed in the neighbourhood 



Fig. -J04. 



T.S. through the spermoderm and aleurone- 

 layer of a germinating Rye-grain. 



