314 LIFE : OUTLINES OF GENERAL BIOLOGY 



fats, and proteins are transported from the green working cells, 

 through the plant, to feed its other cells, to afford material tor 

 immediate growth, or to be stored for subsequent use. 



If the food is to pass from the place of manufacture to the place 

 of usage or storage, it must be able to diffuse through living cells, 

 and this must often involve a change into a state suitable for 

 translocation. Thus cane sugar( C.^H^iO,,) probably travels as some 

 simpler sugar, such as glucose (C6H,j06); fats travel as fatty acids 

 and glycerine, just as in our own body; and proteins probably 

 travel as amino-acids. The change into a diffusible or travelling 

 form is brought about by ferments to which we shall refer later on. 



In a simple plant like a seaweed or a liverwort, the food passes 

 by osmosis from cell to cell, often helped by the bridges of proto- 

 plasm which penetrate the cell walls; but in higher plants there are 

 special paths which effect more rapid transport. The usual view of 

 botanists is that this conducting system is mainly to be found in 

 the soft bast, helped sometimes by the vessels which carry the 

 milky material called latex, so famihar in spurges and dandelions, 

 and so important to us in rubber plants. 



In seed-plants the phloem or bast strands lie to the outside of the 

 xylem or wood-strands, and except in monocotyledons the two 

 systems are separated (should we rather say united?) by the cambium 

 zone of persistently embryonic cells, rejuvenescing and dividing with 

 each growing season. Like the xylem, the phloem forms a con- 

 tinuous system from the stem to the roots below, and to the leaves 

 above: thus it is easy and interesting to tear them apart from a 

 fallen and slightly decayed holly leaf, this yielding two exactly 

 similar half-skeletons. Yet in some cases, at least, they do not usually 

 extend quite so far as the xylem into leaf and root points, since the 

 exit and entrance of water is the more urgent. Then, too, as the 

 wood and bast of any stem so plainly shows, as do also our leaf 

 half -skeletons aforesaid, the wood breaks readily, while the bast 

 fibres only bend, and thus the stem and branches are stayed, 

 essentially as the masts and spars of our ships by the rope-rigging — 

 which wood and bast-fibres have respectively supplied us. This 

 mechanical combination and perfection of the leaf-skeleton, how- 

 ever, incomparably surpasses man's rough skill. 



We are not in this sketch concerned with any full account of the 

 various elements that make up the phloem strands; but outstanding 

 amongst them arc the "sieve-tubes" — living vessels formed from the 

 fusion of the end-walls of a line of cells, and yet with their proto- 

 plasm in continuity through perforated-looking septa, of character- 

 istic aspect. They are thus not fully comparable to the tracheje or 

 wood-vessels of the xylem, which lose their protoplasm and which 

 certainly carry water, and so have usually been regarded as the 

 main transport-lines for the food, especially in its descent from the 



