53G 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Not only does life make them, but locates them when made in \ 

 their respective positions where required. Thus, our food is digested 

 and becomes blood, which traverses the entire person, nourishing even 

 the most minute portion of the body. Like a railway train which 

 stops to deposit parcels directed to various stations, so lime is chiefly 

 deposited by the blood where bones are formed and in the teeth.- 

 Silica is sent to the hair, nails, and teeth, fluorine being also in the 

 bones, as well as the teeth, to make the enamel. Salt is carried to 

 the tears, but not to the mouth. Phosphorus is wanted in the bones , 

 and brain, &c. 



Similarly in plants, sugar is made in fruits, starch is laid up in 

 seeds and tubers, oil is found in seeds ; while silica is deposited in the 

 surface of all grass-stems. Thus we see directivity everywhere at i 

 work. Vegetable products are thus found to be located in definite 

 cells suitable for the purpose. Moreover, cells are of all sorts of 

 shapes and sizes, according to the uses for which they are required. 

 Thus in cork they are roughly cubical ; in fibrous bark, such as makes 

 tow and flax, or shreds of a vine-stem or that of a Clematis, the cells i 

 are of very great length, tough and flexible, tapering to a point at each 

 end; while wood-cells or fibres are short, very thick-walled and firm, 

 for supportive purposes. 



On the surface of a leaf they are colourless and flat, conjointly 

 making a skin or epidermis. Just below that of the upper surface, 

 the cells are rather long and compactly pressed together: hence they , 

 have been called " palisade " cells. It is these which make a leaf | 

 green, for they abound with chorophyll granules. Vessels * composed j 

 of rows of cells, with the partition-walls absorbed, become thickened | 

 in various v/ays by spiral or circular bands, &c. These tubes are | 

 for the more rapid conveyance of water. 



In all these cases the thickening is the result of response to strains, 

 stresses, and weights; so that if stems be artificially bent, the sup- 

 portive tissues increase accordingly in quantity to meet the strain. j 



If we ask why the cell-walls thicken in this way, we can only say 1 

 that the material, as cellulose, is secreted by the protoplasm, the life | 

 of which directs the energy, which places the molecules of cellulose I 

 in such a way as to form a spiral or whatever character the thickening ; 

 may assume. 



Such are a few of the many sorts of plant-cells, each being regularly 

 found in its proper place, and all having some definite use for the 

 plant's requirements. 



How came all this about? What determined these various tissues 

 to be constructed as, and where, we find them? We must go deeper 

 into the matter; for all cells begin alike, and are very minute, mostly 

 quite invisible to the naked eye. But as they grow larger they take 

 on the various forms and structures required, wherever their locality 

 in the plant may be. 



* Easily seen in a piece of oak-wood, looking like pinholes in a surface cut 

 across the grain. 



