RESERVE-CELLULOSE 413 



grain thus quickly becomes exposed to the action of the solvent. The 

 starch-grains in the endosperm of Maize, Barley, llye and other 

 (Iuamineae, also dissolve from the surface inwards, but not simul- 

 taneously at every point. On the contrary, solution begins at a 

 number of more or less widely separated spots. The resulting pits and 

 channels sooner or later intersect one another and ultimately break 

 up the grain into several fragments, which thenceforward dissolve 

 independently. The corroded channels are not of uniform width 

 throughout ; their outline, as seen in optical (longitudinal) section, 

 appears notched or jagged, owing to the fact that the channels remain 

 narrower where they cross the denser layers than where they traverse 

 those which are relatively watery, and hence more easily attacked by 

 diastase. 



It was stated above, that carbohydrate material may be stored in 

 the form of greatly thickened cell-walls. 192 Where this is the case, the 

 storage- tissue often acquires a horny or even a bone-like consistency; a 

 familiar illustration is provided by the endosperm of Phytelephas macro- 

 carpa, which constitutes the so-called "vegetable ivory." Various other 

 Palms (e.g. the Date-palm), many LlLlACEAE (Omithogalum, Fritillaria) 

 and Iridaceae and certain other Monocotyledons are provided with a 

 similar type of endosperm. In certain Leguminosae carbohydrate 

 substances are stored in the form of mucilaginous layers, in the walls 

 of endosperm cells. In other members of the family the membranes 

 of the cotyledonary cells are thickened for the same purpose ; a 

 similar arrangement is found in Impatiens Balsamina, Paeonia 

 officinalis, and Tropaeolum. Finally, it has been shown by Schaar that 

 the thick-walled cells of which the bud-scales of the Ash are 

 composed, represent a store of carbohydrate material destined for the 

 nutrition of the young shoots in spring. 



The carbohydrates which are stored in the form of thickened cell- 

 walls, may all be included in the comprehensive category of reserve- 

 cellulose. Chemically, they belong, according to E. Schulze, to the 

 class of hemi-celluloses, which are distinguished by the fact that they 

 can be hydrolysed with relative ease (cf. p. 47). The most widely 

 distributed of these hemi-celluloses are the mannans and galactans, the 

 former being especially characteristic of Palm-endosperms, the latter 

 of Leguminous seeds. A peculiar form of reserve-cellulose, which 

 resembles starch in giving a blue reaction with iodine alone [i.e. 

 without previous treatment with acid], is known as amyloid ; it occurs, 

 for example, in Impatiens Balsamina, in Tropaeolum majus, and in 

 many Primulaceae. The types of reserve-cellulose which have been 

 mentioned, certainly do not include all the carbohydrates that belong 

 to this category. It is most probable that such hemi-celluloses occur 



