248 



KNOWLEDGE 



[NOVEMBEB 1, 1890. 



sulphur, and probably phosphorus, enter into Its constitu- 

 tion. That a plant may be able to elaborate its proto- 

 plasmic material, it must be supplied with those elements. 

 They have for this reason been termed the I'ssmtial demctits 

 of plant food. But in addition to these the presence of 

 potassium, magnesium, and u-on has been found neces- 

 sary for the healthy and vigorous development of the 

 plant. The question now arises. From what sources does 

 the plant obtain these elements ? The answer is, From 



slom 



stoma. 



Fig. 2. — Epidermis of — a, Hoya : li, Scolopendrium vnlgare : 

 c, Lilium candidum ; d, Brassica oleracea. Cabbage ; e. Iris. 



the soil and from the air. The air furnishes it with carbon 

 and oxygen ; the other elements are derived from the 

 soil. The mode of obtaining carbon and oxygen we have 

 just described ; but, besides, in the aii- there is a great 

 quantity — 4th part by volume— of free oxygen, so that 

 for the purpose of respiration the plant can obtain it, so to 

 speak, ready-made. It seems very strange, but still the 

 statement is supported by numerous experiments, that 

 although there is as much as 80 per cent, of free nitrogen 

 in the atmosphere, plants do not make use of iti in. 



preference taking it in the combined forms from the soil. 

 The organs which are the absorbents of carbon and oxygen 

 are the leaves, and also the stem when it contains green 

 colouring matter ; while by means of numerous hairs, 

 which are developed at a little distance behind their 

 growing point, the young roots absorb the nutrient mate- 

 rial from the soil. Before a substance can pass into a 

 plant it must be in solution in water ; this applies to gases 

 as well as to solids. The soluble salts of the soil are pre- 

 sent there in very dilute solutions, and it is only as such 

 that the plant is capable of receiving them. Taking into 

 consideration the exceedingly dilute state in which the 

 salts are absorbed, and the large quantity of the elements 

 which compose them required by the plant to enable it not 

 only to increase in size, but even to live, it will be seen 



f^orenctujmo resbifal-ory eavitu 

 D. 



Stoma 



I Cells 



Fig. 3. — a. Transverse section of stomatal apparatus of Crassnia 

 napus ; h. Iris ; c, Lilinm candidum, White Lily ; d, Equisetum 

 arvense, Horse-tail. 



that a gi-eat volume of liquid must be taken in by the 

 roots of plants. The absorbed liquid forms what is gene- 

 rally termed the sap. It travels up the stem chietly by 

 way of the wood to the chlorophyll-bearing cells, where it 

 unites with the carbon and oxygen obtained by these cells 

 from the air. Only a very small quantity, however, of the 

 absorbed water is required by the plant. The superfluous 

 water must be given off to promote fresh absorption by 

 the roots. This giving olf of water is termed transpiration , 

 and it is the function of the stomata to regulate this pro- 

 cess. The most active transpiration takes place when the 

 stomata are open, and it is also at this period that carbon 

 assimilation is proceeding with the greatest vigour. The 

 two processes are thus closely connected the one with the 

 other, and the mechanism of the stomata is wonderfully 



