PLANT CELLS. 



663 



Fig. 741. Vascular tissue. 



fluid can enter into the 



As every plant, whether small or great, is only an aggregate of a great 

 number of cells, so, also, the life of a plant is 

 nothing else but the sum of the activities of all 

 the cells of which it is composed. The special 

 province of the cells is to receive from the soil or 

 atmosphere the water necessary for the various 

 vegetative purposes, together with the nutritious 

 materials dissolved in the watery and aerial fluids, 

 and to circulate them through the whole body of 

 the plant. The circulation within a plant is 

 not carried on through the agency of tubular 

 channels, but only by the passage of sap in all 

 directions from one cell to another. 



Since the cells have no openings, it is some- 

 what difficult to understand in what manner the 

 plant from without, and by what means it can inwardly pass from cell to cell. 

 This phenomenon, however, is dependent on the peculiar quality both of 

 vegetable and animal membranes and fibres viz., that they are permeable by 

 many fluids, without being dissolved by them. Experiments show that this 

 permeative action is carried on in accordance with definite laws. When two 

 fluids of unequal densities as, for example, an aqueous solution of sugar and 

 mere water are separated from each other by a diaphragm 'of pig's bladder, 

 we perceive a constant tendency on both sides to restore the equilibrium in 

 the density of the two fluids. A portion of the water penetrates the bladder, 

 mixing with the solution, and a portion of the latter finds its way to the 

 former by the same medium. In this experiment one important fact is to 

 be observed viz., that the lighter fluid always passes through the separating 

 medium more rapidly to the denser than vice versa ; consequently, in this 

 experiment more of the water passes through the bladder to the saccharine 

 solution than of the latter to the water. This permeative capability of the 

 tissue of vegetables and animals is called endosmose. 



The cells both circulate the sap and alter its corfdition, so we find 

 differing substances in the same plant. The cell as described creates 



new cells, and the force with which the 

 sap rises is rather greater than the pressure of 

 the atmosphere. 



The vascular, or fibrous tissues, are illus- 

 - trated in the margin (fig. 741). They usually 

 contain air. Some plants have no vascular 

 Fig - 742 - tissue, and are termed cellular plants such as 



mushrooms, fungi, mosses, and seaweeds. Many contain both -tissues, and 

 these are the more highly developed kinds. 



Sometimes we find a milky juice in plants. This is called latex, and caout- 

 chouc is always present in it. This juice is contained in tiny tubular vessels, 

 which have their origin in the new cellular tissue of the lactiferous plants. 



