422 STORAGE SYSTEM 



within a single tissue. The two remaining types, on the contrary 

 (5 and 6), are characterised by the segregation of the two main classes 

 of reserve-materials, the protein compounds being separated in great 

 part at any rate from the non-nitrogenous substances. In the fifth 

 type (5) nitrogenous and non-nitrogenous materials occur side by side 

 in the same cell ; in the sixth and most advanced type, however 

 (6), they are deposited in two perfectly distinct tissues. As might be 

 expected, the six principal types of storage-tissue that we have 

 recognised, are connected with one another by a number of 

 intermediate forms. 



Some attention must now be given to the shape and arrangement 

 of storage-cells, and to the characters of their walls. 



In the case of starch-storing tissues, the cell- walls are frequently 

 very thin, and quite devoid of pits (endosperm of Grasses, bulb-scales 

 of Oxalis esculenta). Wherever, on the other hand, the walls are even 

 moderately thick, numerous pits are present, which facilitate the 

 filling and emptying of the storage- cells. These pits are very minute 

 in the cotyledons of the Oak, but attain a considerable size in the 

 Horse-chestnut, and also in the Bean and other Leguminosae. When 

 the non-nitrogenous reserve-materials are stored in the form of oil, the 

 walls of the storage-cells are generally thin and unpitted. Where the 

 secondary thickening layers of a wall consist of reserve-cellulose or 

 amyloid, they may be quite homogeneous, except for a highly 

 refractive limiting pellicle, as in the endosperm of Ornithogalum 

 umbellatum and Phytclephas macrocarpa ; in other cases (e.g. endosperm 

 of Fritillaria imperialis, Tig. 165 c) they exhibit very obvious 

 stratification. Such thickened walls are, of course, always plentifully 

 provided with pits which are deep and, as a rule, also fairly wide. 

 Sometimes these pits are dilated at their lower ends in a funnel- or 

 trumpet-shaped manner (Fig. 165 d); this arrangement serves to 

 enlarge the area of the limiting membrane which is often itself 

 somewhat thickened and thus to accelerate diffusion. In a few 

 cases, according to Strasburger and Tangl, the same result is achieved 

 in a different way, the limiting membranes being perforated by a 

 number of protoplasmic connecting threads (Ornithogalum umbellatum, 

 Phoenix clactylifera). Special interest attaches, in this connection, to 

 the very narrow perforations of the endosperm-walls discovered by 

 Tangl in Stryelrnos Nux vomica; these are not confined to the pit 

 membranes, but also traverse the thicker portions of the wall. 

 Whether such direct paths of communication serve for the rapid 

 distribution of solvent enzymes, or whether they are utilised in the 

 transportation of the products of solution, is still an open question. 



In the less specialised types of storage-tissue, the component 



