HOW PLANTS GROW. 167 



DIFFUSION OF LIQUIDS 



with each other is thought to assist in the movements of sap. Thomas Gra-' 

 ham divided substances into crystalloids, such as salt, sugar, etc., and colloids 

 such as starch, gum, and gelatine. Crystalloids move freely through mem- 

 branes to mix with colloids, but the reverse is not the case. 



There is no fact better established in vegetable physiology than this : that 

 the sap of plants goes to the green leaves or surface of the plant and there 

 becumea changed or re-organized into material fit to nourish the plant. 



Over 100 years ago, in trees the sap was thought to go up in the young wood 

 and descend in the cambium layer. Numerous facts seemed to prove this. 

 A thread or wire tied closely about the tree causes a bunch or ring to form 

 above it. 



In coniferce (pines, spruces, cedars, larches, etc.), the wood cells are all 

 nearly alike. There are no vessels or ducts. In such plants the sap ascends 

 in the wood cells, passing from cell to cell through vast numbers of partitions 

 in its course to the leaves. In all woody plants of our climate except the 

 coniferce, there are numerous ducts or vessels, long tubes much larger than 

 the wood cells. The older botanists believed (and some of the later) that sap 

 ascends in these vessels, while most recent botanists believe the sap ascends 

 through the woody tissue. The vessels are sometimes full of sap and some- 

 times full of air. 



If a limb containing good leaves is cut off and placed in an alumed decoc- 

 tion of logwood or magenta, the dye will ascend more or less rapidly, accord- 

 ing to the evaporation of the leaves. A cross section of the limb near the 

 liquid shows all the wood cells full of colored sap and the vessels empty. A 

 cross section higher up at a certain time shows a few wood cells full of colored 

 water, and these are all in the form of a ring just about a vessel. Here too 

 the vessel is empty. 



Herbert Spencer concludes that the fluid goes up in the vessels and passes 

 sideways to the woody tissue. The mechanical and physical forces mentioned 

 probably aid in the circulation or movement of the sap, but they alone do not 

 seem to satisfactorily explain the whole subject. In petals, for example, or 

 any parts of the flower or leaves which are not green, I have hundreds of times 

 noticed under the microscope that some of the cells may be filled with a clear 

 liquid, and others next to them, separated only by a cell wall, are filled with a 

 liquid which may be deep blue or red or violet. How is this if sap is under 

 the control of osmose? More than this, orange-colored flowers appear so be- 

 cause some cells are full of a red fluid while the cells next to them are full of 

 a yellow fluid. How can we explain the fact that liquids pass up and down 

 long drooping branches of elms and willows? 



In the turnip and beet the crude sap goes from the roots to the leaves, and 

 then some of the nourishment goes back to thicken the root, which stores up 

 food for future use, as a bear gets fat in autumn and lives on fat all winter. 

 In spring the nourishment in the root of the turnip goes again into the stem 

 and leaves above. By culture and selection a wild, worthless plant on the 

 coasts of Britain has changed into several forms, and is now known as cabbage, 

 kale, broccoli, Brussels-sprouts, cauliflower and kohl-rabi. In the cabbage 

 the nourishment is partly stored in the stem, but largely in the leaves; in 

 Brussels-sprouts, in the buds or branches; in cauliflower, the flower stems; in 

 kohl-rabi, the stem is gorged with food. Here the same species is made to 



