466 MECHANISMS FOR CONVEYANCE TO AND FRO. 



described. It has, moreover, been stated that numerous plants exist which 

 consist only of single green cells; that others, which are multicellular, exhibit 

 in all their cells the same shape and grouping of the chlorophyll-corpuscles; 

 and finally, that in most seed plants, a division of labour has taken place in 

 each plant, so that certain cells only are provided with chlorophyll, while others 

 are always destitute of it. Many parasites are quite free from chlorophyll, and 

 consequently are unable to decompose carbonic acid and to manufacture organic 

 materials. They are obliged to suck these up from their hosts. Closely connected 

 with these are cases of symbiosis, in which plants possessing, and plants devoid 

 of, chlorophyll enter into partnership, and in which the latter receive in exchange 

 certain freshly-manufactured organic substances from the former. The conclusion 

 of this long series is formed by the saprophytes, devoid of chlorophyll, which 

 derive their organic materials not from living green plants, but from dead animal 

 or vegetable bodies, and from the lifeless organic substances arising out of plants 

 or animals. In the green unicellular plants, as, for example, in the Desmidieae, 

 two sj)ecies of which are illustrated in fig. 25a, i, k, all the various combinations, 

 arrangements, and separations of the atoms which lead to the formation of sugar, 

 starch, cellulose, chlorophyll, albumen, &c. are accomplished within a single cell; 

 and these minute plants furnish evidence that the manifold changes of the 

 materials connected with growth and construction can occur side by side at the 

 same time and in limited compass. In order to be able to demonstrate this, it 

 must be assumed that each tiny protoplasmic mass, which forms the living body 

 of the single cells, is made up of various portions to which are assigned different 

 functions. One breaks up carbon and forms carbohydrates; another takes up 

 these carbohydrates and forms albumen from them; and yet another transforms the 

 sugar into cellulose and builds up the cell- wall. 



With this assumption, however, is necessarily connected the further assumption 

 of a transportation of materials. In unicellular Desmidiese, the path which the 

 sugar produced in the central chlorophyll-bodies has to travel in order to reach 

 the periphery of the cells, is perhaps only two or three thousandths of a millimetre 

 long; it is, however, a measurable distance, and therefore there is such a thing as 

 conveyance and removal of sugar in cells of Desmidiese. The transportation is 

 without doubt again carried on by certain portions of the protoplasm, and perhaps 

 the manifold strands which are observed in the substance of the protoplasm are 

 associated with this. In multicellular plants the road which the materials 

 have to follow, in order to reach their destinations, though frequently limited 

 to the space of a single tiny cell, is often represented by a long row of cells. 

 This is especially the case when certain functions are assigned to the different 

 cells of a plant, as happens in many spore-bearing plants, but still oftener in 

 seed-plants. The materials formed in the green leaves of a moss, if they are 

 to be employed in the construction of the spore-capsule and in the production 

 of spores, must be transported from cell to cell, to the archegonium situated on 

 the moss stem — a road which varies according to the species from some millimetres 



