ALEURONE-LAYER 



505 



process must have been purely incidental in relation to the ordinary 

 activities of the leaf. Subsequently the adaxial hydathodes became 

 transformed into digestive glands, and the plant was thus enabled to 

 make better use of the advantages resulting from the carnivorous habit. 

 As a final result there ensued the division of labour between the sessile 

 and stalked glands to which attention has already been directed. 



In all probability the digestive glands of other carnivorous plants 

 have had a very similar evolutionary history. 



2. The digestive glands of embryos?*' 



As Sachs has clearly explained, the mobilisation of reserve- 

 materials which takes place during the germination of seeds, 

 serves not only to render the various stored up plastic materials 

 soluble, and thus suitable for translocation, but also to transform 

 them into compounds which can be immediately utilised by the 

 growing tissues. It is probable that this 

 mobilisation is always brought about by 

 the action of enzymes. Hansen is there- 

 fore, no doubt, justified in terming the 

 whole process of mobilisation " diges- 

 tion," by analogy with the corresponding 

 processes which take place in animals. 

 In the majority of cases the protoplasts 

 of the storage-cells are themselves capable 

 of secreting sufficient quantities of the 

 appropriate enzymes. Sometimes, how- 

 ever, particularly when the endosperm of 

 a rapidly germinating seed is being 

 emptied, the storage-cells are assisted in 



their work of enzyme-secretion by special digestive glands, which 

 then become responsible for the solution of the bulk of the re- 

 serve-materials. Researches upon this subject have hitherto been 

 restricted to the Geamineae. When germination first begins in these 

 plants, the absorbing tissue of the scutellum secretes diastase, which 

 attacks and dissolves the starch-grains in the adjacent layer of the 

 endosperm. Soon afterwards, however, diastase also exudes from 

 the so-called aleurone- layer, a tissue which was formerly assigned to 

 the storage-system, but which, in the author's opinion, really represents 

 a glandular digestive organ. 



In the resting seed, the aleurone-cells, which constitute the outer- 

 most layer of the endosperm, contain numerous aleurone-grains (Fig. 

 203, hi.), which, in spite of their minute size, possess the usual struc- 

 ture (including the presence of a globoid). Each cell further contains a 



Fig. 203. 



T.S. through the peripheral tissues of 

 a resting Wheat-grain ; s, spermoderm 

 (pericarp and testa) ; ki, aleurone-layer ; 

 z, cells of the starchy endosperm, x 300. 



