IX Leaves 167 



verdure returns on the addition of a drop of ferric 

 chloride. It is also possible to prepare culture solutions 

 in which all necessary constituents are present, and thus to 

 grow many plants without soil. In many leaves it is easy 

 to demonstrate the presence of oxalate of lime — we can 

 taste it in the wood-sorrel and the sour-dock — and this salt 

 seems to be a by-product of chemical reactions which go 

 on in the leaf in which the nitrates and sulphates absorbed 

 from the soil are utilised in the formation of those complex 

 organic substances which chemists call proteids. 



If we put the cleanly -cut stalk of a primrose -leaf 

 (still better, that of the Chinese primrose {P. si?te7isis)^ 

 so common and free -flowering in our greenhouses) into 

 our mouth, and keep the blade of the leaf immersed in 

 water, we can blow air through the stalk into the blade and 

 thence into the water. It issues from the leaf through 

 minute apertures, most numerous on the lower surface. 

 They are readily seen under the microscope, especially on 

 a leaf like that of the Iris, from which it is easy to peel off 

 a film of transparent skin. They are of great importance 

 in the life of the leaf, and are called stomata. To reverse 

 the above experiment, the leaf- stalk should be inserted 

 through a perforated india-rubber cork into a beaker half 

 filled with water ; the base of the stalk should be well 

 immersed in the water. Then if through a glass tube also 

 perforating the india-rubber cork we suck out air from the 

 upper half of the beaker, the altered pressure induces a 

 passage of air through the stomata of the leaf into the 

 blade and down the stalk into the water, through which it 

 issues as a stream of bubbles. 



The recognition of these openings or stomata makes the 

 leaf more intelligible to us. They lead into spaces within 

 the substance of the leaf, and with a microscope it is easy 

 to prove that they are open when the plant is in full vigour. 



