230 THE PLANT. X. 



that the protoplasm is continuous from cell to cell, the con- 

 tinuity being maintained by fine threads passing through 

 minute perforations in the cell walls. Through these small 

 apertures, diffusion of dissolved matter in the protoplasm itself 

 will take place at a rate probably far greater than is propor- 

 tional to their area. The apertures, in fact, as suggested by 

 Brown and Escombe,* play a similar part to the stomata in 

 the leaves in promoting diffusion without interfering with the 

 structural advantages of the cell walls. f 



2. The Stem from our present aspect may be regarded as 

 the mere means of communication between the roots and the 

 leaves. It, however, serves many other purposes, e.g., often as 

 a receptacle for reserve material or for excreted matters, and to 

 some extent, when green, as an assimilative agent. 



3. The Leaves are the seat of the most important chemical 

 changes occurring in the plant. It is here that the reactions 

 characteristic of vegetable life mainly take place the forma- 

 tion of carbohydrates from carbon dioxide and water, of amides 

 and albuminoids from the same constituents and the nitrates 

 and sulphates taken in through the roots. They also fulfil 

 another most important function as a means by which the 

 water absorbed by the root may be got rid of by evaporation. 

 This process of transpiration, as it is called, takes place at a 

 rate which depends upon several circumstances, among others 

 upon the hygrometric state of the atmosphere round the leaf. 

 The absorption of carbon dioxide and the evaporation of water 

 take place mainly through the minute openings on the exterior 

 of the leaf known as stomata. Each stoma is provided with 

 two guard cells, by the varying turgescence of which the size 

 of the opening can be regulated. If the guard cells become 

 flaccid, as they tend to do by excessive loss of water by trans- 

 piration and also in the dark, they more nearly close the 

 opening the stoma between them and so lessen the passage 

 of water vapour outwards and of carbon dioxide inwards. The 

 space below the epidermis of a leaf is surrounded by cells 



* Phil. Trans. 193(1900), 2SO. t v. p. 233. 



