8 PLANT LIFE ON THE FARM. 



euch, but there are myriads of other plants which consist 

 of but one cell ; in such a case the cell is the plant, the 

 plant is the cell. Now this is important, because it shows 

 us that all the processes of life can be, and often are, 

 carried on in one cell only, that is, by one fragment of 

 protoplasm. Where the fabric becomes more complex, 

 one cell is more or less dependent on another, but still 

 there is always a measure of independence left to each 

 individual cell. Were it not so, the scythe of the mower 

 or the grazing of the sheep, by destroying a portion, would 

 kill the entire plant. 



It follows that the life-history of a plant is, in essence, 

 the life-history of protoplasm and of its covering, the cell- 

 wall ; and hence it is that the microscopist or the chemist 

 in the laboratory studying what goes on in isolated cells, 

 placed as far as possible under uniform conditions, is really 

 adopting the best means of investigating what takes place 

 in the entire plant ; a circumstance which the "practical 

 man," so called, compelled to work in the field under such 

 very different, more complex and much less definite 

 conditions, finds it difficult to realise. 



Conditions of Diffusion. Diffusion, it will readily be 

 understood from what has just been said, is not equal or 

 alike in all cases ; it depends upon the extent to which the 

 two liquids are diffusible, upon their different densities, 

 upon temperature, and a variety of other conditions. So, 

 in the case of osmosis, we have not only the nature of the 

 two fluids to consider, but their relation to the membrane 

 that separates them. The membrane may be much more 

 permeable to one of the two fluids than to the other. 

 Thus, in the case of a living cell, the membrane or wall is 



