88 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



brane, thus reaching the plasma, and finally through the internal plasmatic 

 membrane bounding the vacuole. A naked plasma mass or gymnoplast 

 may take in solid, insoluble, or undissolved substance, and thus the plas- 

 modia of Myxomycetes may be observed to ingest particles of sand, 

 spores, oildrops, &c., and after a time to reject them (Sect. 19). It is 

 owing to the semifluid consistency of protoplasm, and its power of im- 

 mediately closing a wound or opening made in it, that the ingestion and 

 excretion of solid bodies are possible. Indeed, the protoplasm of a closed 

 cell may cither ingest crystals, oildrops, &c. from the cell-sap or excrete 

 them into it. 



Organized structures may even penetrate protoplasts provided with 

 a cell-wall, as, for example, when plants are infested with the hyphae of 

 parasitic fungi or attacked by certain bacteria. When, however, solid 

 substances external to the cell are rendered soluble by the action of 

 excreted acids or ferments, the actual process of absorption is of the 

 normal character, since the substances are imbibed in solution. The roots 

 of both higher and lower plants may actually exert a solvent influence of 

 this nature by means of acid excretions, while the developing embryo, and 

 parasitic and saprophytic plants as well, may excrete solvent enzymes. 

 There can be no doubt that the neighbouring cells of a tissue may affect 

 one another in a similar manner, and possibly in other ways also, while the 

 fine plasmatic threads which connect the different protoplasts with one 

 another render an additional means of communication available for the 

 transference of food material or the conduction of stimuli. 



As every active living cell is saturated with imbibed water, and as 

 even those seeds, mosses, and lichens which can withstand desiccation 

 remain dormant until they are supplied with water, the penetration of 

 gases into dry organized structures is of subordinate physiological im- 

 portance. Indeed, even the walls of dead cells contain more or less 

 imbibed water, so long as they are surrounded by living tissues, and it is 

 only in peripherally situated dead tissues that the percentage of imbibed 

 water may be reduced almost to nil. Gases pass in solution through 

 a cell-wall saturated with water by a process of diosmosis, just as they 

 do through a soap bubble, and the same general principles hold good for 

 such gaseous interchange as for the diosmosis of other dissolved substances. 

 The movements of gases through dry organized bodies will be discussed 

 later in giving a special account of the gaseous interchanges. 



The diosmotic properties of cell-walls, protoplasts, &c., are specifically 

 different : thus suberized or cuticularized cell-walls scarcely allow water 

 to pass through them at all, or only with great difficulty. This peculiarity 

 is attained by the cell-wall becoming more or less completely impregnated 

 with fatty, waxy, or resinous substances, and a piece of paper that has 

 been partially or entirely saturated with wax, fat, or oil, gives a fairly 



