TYPES OF STORAGE TISSUES 421 



(ii.) Reserve-materials contained partly in the cell-cavities and 

 partly in the cell-walls. 



5. Aleurone-grains and fatty oil (in the cell-cavities) plus 



reserve-cellulose (forming the thickening layers of the 

 cell- walls) : Cotyledons of Impatiens Balsamina ; endosperm 

 of Phytelephas macrocarpa, Phoenix dactylifera, Goffea 

 arabica, Ccratonia Siliqua. 



B. Reserve-materials distributed between two different tissues. 



6. One portion of the non-nitrogenous reserve-material stored 



in the thickened mucilaginous walls of endosperm cells, 

 the cavities of which are reduced to narrow crevices con- 

 taining no appreciable quantity of plastic substance ; the 

 rest, comprising fatty oil and sometimes also some starch 

 deposited in association with aleurone-grains in the cells 

 of the cotyledons : Trigonclla Focimmt graecum, Trifolium 

 pratense, Medicago. 



In a number of seeds the outermost layer of the endosperm is 

 specialised in a peculiar manner : it is composed of rather short, 

 prismatic thin- or moderately thick-walled cells, which contain only 

 aleurone-grains and oil, even when carbohydrates in the form of starch 

 or reserve-cellulose predominate in the rest of the endosperm. Such 

 an aleurone-layer, as it is termed, is particularly well developed in 

 Grasses ; here, however, in the author's opinion at anyrate, it does not, 

 properly speaking, belong to the storage-system at all, but represents a 

 glandular tissue which secretes diastase at the time of germination. 

 Further investigation is required, in order to decide whether the 

 aleurone-layer acts as an enzyme-secreting tissue in other families also, 

 notably among the Leguminosae or whether it merely represents a 

 specially differentiated portion of the storage-system. 



When the various forms in which reserve food material is stored 

 are set forth in tabular form, we see quite clearly that the several 

 types of storage-system exhibit a gradually increasing perfection of 

 adaptation, associated with an increasing division of labour. In the 

 first of the types distinguished above (1), the elements of the 

 storage-tissue do not differ morphologically from ordinary active 

 parenchymatous cells. Storage-cells of the second type (2), may be 

 said to combine the characteristics of active and of resting elements to 

 a certain extent. In the third and fourth (3 and 4), all the plastic 

 substances present have assumed forms which are associated with the 

 quiescent condition of the cell. In all the cases so far considered the 

 whole of the reserve-material is deposited in the cell cavities and 



