ii2 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



In Myxomycetes, amoeboid movements suffice to mechanically intro- 

 duce and expel foreign particles from the substance of the protoplasm. The 

 same is also the case in protoplasts enclosed by a cell-wall, and since, in such 

 cells, the protoplasm is relatively small in amount, all unused solid substances 

 usually collect in the cell-sap. These include calcium oxalate, as well as dead 

 fragments of protoplasm. Hence it follows that there must be a definite 

 reason for other solid particles remaining permanently imbedded in the 

 plasma. This applies not only to actual protoplastic organs, but also to 

 certain algae which live symbiotically in the bodies of many Infusoria, 

 Radiolaria, &c., but which, when they are ingested by a plasmodium, are 

 always rejected sooner or later. Starch grains and chlorophyll grains, when 

 ingested, meet with the same fate ; so that, as soon as starch grains are 

 freed from the chromatoplasts which have produced them, they will almost 

 certainly be transferred by the plasma to the cell-sap. Moreover, there is 

 little doubt that stimulatory influences may come into play, causing certain 

 substances to be ingested with greater avidity than others, as, for example, 

 by means of chcmotactic attraction. In such cases, the organism is 

 evidently able to directly select the food material it prefers 1 . 



On the other hand, small organisms, by means of their own activity, 

 aided or unaided by an attractive stimulus, are able to penetrate protoplasts 

 and thus to be ingested. This takes place when Bacteria, fungal hyphae, 

 Myxamoebae, Plasmodiopliora, &c., penetrate plant-cells. In these and other 

 cases, such as the process of fertilization by means of a pollen tube, the pene- 

 trating body must first make its way through the cell-wall. (See Sect. 65.) 



Further research is needed to determine the means by which the 

 necessary intimate approximation is assured, as well as the causes inducing 

 both active penetration by foreign bodies, and their passive ingestion. The 

 mechanism involved may, moreover, be of complicated character, and may 

 in certain cases even involve special stimulatory reactions. We are here 

 dealing with important physiological relationships, of which only a few 

 have received a causal explanation, and these will be mentioned later. 

 We may however state in illustration, that the antherozooid of a fern 

 is attracted to the ovum by the chemical attraction exerted by malic acid 

 derived from the contents of the archegonium, and thus antherozooids 

 may penetrate the archegonia of widely different species : they fuse, however, 

 only with the ova of their own species. 



SECTION 20. Translocation from cell to cell. 



We have already seen that the fundamental problems connected with 

 the exchange of substance are to be studied in relation to the mechanism 



1 Examples of such processes are given by de Bary, Pilre, 1884, p. 481, &c. The same pheno- 

 mena are shown by leucocytes, and these may also pass through membranes, walls of capillaries, &c. 



