THE LIFE -HISTORY OF PLANTS. 



41 



osmosis or endosmosis that the water outside a cell 

 passee thi'ough the membrane into its cavity. And so 

 if the thin watery contents pass out by exudation or 

 by pressure from one cell, they may pass by osmosis 

 into another adjacent cell with thicker contents. 

 This passage of fluid from cell to cell is manifested 

 principall}^ by the " cell-sap," which is of a thin 

 watery character. 



The "protoplasm" differs from the membranous 

 cell-wall, and from the 

 cell-sap, in being- a thick 

 viscid fluid containing a 

 proportion of nitrogen, and 

 sometimes a little sulphur 

 and phosphorus, in addition 

 to the carbon, oxygen, and 

 hydrogen, of which cell- 

 membrane is composed. The 

 protoplasm is by far the 

 most important part of the 

 plant. Though seemingly 

 a mere translucent, jelly- 

 like substance, it is the seat 

 of all the life-work done in 

 and by the plant. "SVith it 

 and by it the plant lives ; 

 without it, it is dead. The 

 essential part of animal 

 structure is practically iden- 

 tical with vegetable proto- 

 plasm, so that in this 

 wonderful combination all 

 the phenomena of life, 

 vegetable or animal, are 

 manifested. Until recently, 

 it was supposed that (with a 

 few exceptions) the proto- 

 plasm of one cell, in the 

 adult condition, was abso- 

 lutely cut off from that in 

 the adjoining cell by 



means of the walls of the two cells; but it has 

 now been shown that there is frequently a pas- 

 sage of minute thi-eads of protoplasm from one cell 

 to another through apertures in the cell- walls of 

 extreme minuteness. The passage is revealed by 

 treating the protoplasm with some chemical re-agent 

 which stains it, and thus renders the threads visible 

 lender the highest powers of the microscope. In 

 this manner, the "continuity of the protoplasm" has 

 been demonstrated in so many instances as to lead to 

 the inference that it is a general phenomenon ; and, 

 if this prove to be so, many other points of physio- 

 at present obscui-e, wiUbe rendered intelligible. 



Conditions of Growth —Growth, then, we 



Fig. 32.— Endosmometer, 



have seen, essentially consists in the increased for- 

 mation from the old material of protoplasm and 

 ceU-membrane, and to insure this new formation 

 certain conditions must be complied with, and com- 

 plied with, too, at the proper time. These conditions 

 are, a favourable amoimt of temperature and light, 

 and an adequate provision of suitable food, especially 

 of water. 



As to the amount of heat required, that varies 

 within wide limits. Arctic 

 snows do not absolutely 

 banish vegetation ; tropical 

 heat does not destroy it 

 (provided water be sup- 

 plied) . On the other hand, 

 excessive cold or excessive 

 heat will either of them 

 destroy the protoplasm. 

 Each plant thrives best at a 

 certain temperature, and is 

 destroyed by too high or too 

 low a temperature, the exact 

 degree being determined 

 only by practical experience 

 in each case. Even the 

 separate functions of the 

 plant are regulated in the 

 same way, as has been 

 mentioned at an earlier 

 page (\ol. i., p. 23). The 

 general principle is easy to 

 understand, but when the 

 gardener wishes to reduce 

 the principle to practice in 

 individual cases, nothing 

 but experience and obser- 

 vation can guide him satis- 

 factorily, because " circum- 

 stances alter cases "so very 

 materially. Fortunately, in 

 most cases the range is so 

 wide that comparatively little difficulty is expe- 

 rienced; but in other casjs great nicety is demanded, 

 and in plants which have but little power of adapting 

 themselves to circumstances, in forcing-operations, in 

 the proper ripening and colouring of fruits, and the 

 like, practical instruction and personal observation 

 are matters of primary importance to the cultivator. 



"We may, however, point out that while a know- 

 ledge of the climatic conditions may be, and often 

 is, of the highest importance to the cultivator of a 

 plant derived from a particular locality, yet it 

 does not necessarily follow that the gardener will 

 obtain the best results by imitating, or even at- 

 tempting to imitate, in this country, those parti- 

 cular natural conditions. In fact, plants are very 



