186 BOTANY part i 



living in a mediiiin containing some 3 per cent of common salt and but little 

 Ijotassium salts, these cells, nevertheless, absorb relatively little common salt, but 

 accumulate potassium salts together with phosphates, nitrates, and iodine. Thj 

 same power allows Pcnicillium glaucum to flourish in a food-solution saturated with 

 copper sulphate, since only the nutritive substances, and not the poisonous 

 sulphate of copper, are allowed to enter the cells. 



Every substance to which the limiting layers of the protoplasm 

 are permeable must ultimately reach the same concentration in the 

 vacuole as in the solution outside the cell. Practically it often enters 

 in much greater amount than this. Thus, for example, only a trace 

 of iodine is present in sea-water, but may be accumulated in such 

 quantities in sea-weeds for these to become a source from which it is 

 commercially obtained. The cell has not only, a selective power, but 

 is also able to store up materials by converting them into insoluble or 

 indiffusible forms. 



A local accumulation of sugar, or of other soluble reserve materials in fruits, 

 seeds, bulbs, or tubers, would not be possible without such transformation, for the un- 

 controlled osmosis would lead to a uniform distribution of the diosmosing substances 

 throughout all the cells of the plant. If the easily diffusible substance is changed 

 in certain cells or tissues into one which does not diosmose, the diosmosing 

 substance can flow continuously into these cells, and the substance into which it is 

 transformed is stored up in the cells. This process is particularly well seen in 

 transformation of the osmotically carried glucose into insoluble starch. Since the 

 glucose passing into tubers and seeds is continuously transformed into the in- 

 soluble polysaccharid starch, fresh supplies of glucose pass into these cells, and a 

 large store of carbohydrate can be accumulated. 



Gases. — There are no air-filled spaces or canals in either the 

 cell-wall or the protoplasm by means of which gases might pass into 

 the cell-cavity. On this account gases can only be absorbed in so far 

 as they are solul:)le in the cell-wall or protoplasm, or rather in the 

 water of imbibition which saturates these structures. The dissolved 

 gases behave like other dissolved substances, and diffuse into the cell. 



Passage Outwards of Substances. — Substances pass out from the 

 cell according to the same laws which govern their entrance into the 

 cell. 



B. Absorption of Substances in the Multicellular Plant 

 Translocation of Substances in the Plant 



1. Principles of the Translocation of Substances 



It was assumed in the foregoing consideration of the absorption 

 and giving up of substances, that the plant consisted of a single cell 

 surrounded with water. The matter is not so simple when a cell is 

 only in contact with water or with a damp substance on one side, 

 while the other is in contact with the air. The exchange of 



T 



