Cultural First Principles. 



By G. S. BOULGEE, F.L.S., F.G.S., late Professor of Natural History in the 

 Agricultural College, Cirencester. 



VI. Green Leaves. 



{Continued from page 608.) 



The leaves are at once the lungs, the stomach, and the heart of" a 

 plant. A cultivator, then, who neglects the physiology of the leaf 

 is comparable to a physician who leaves respiration, digestion, and 

 circulation out of consideration in diagnosing a disease. 



A leaf is a lateral branch of the vascular tissue of the stem-system, 

 with secondary branches united by a web, so to say , of cellular tissue, 

 the whole being covered by an epidermis. It is roughly comparable 

 to the digits of a duck's foot, united by the web of skin. The main 

 vascular bundle passes along the stalk or " petiole," and the centre of 

 the blade of the leaf or " midrib." The secondary branches are termed 

 "veins;" their arrangement "venation." These constitute the' 

 " skeleton" of the leaf, which so often remains after the decay of the 

 cellular portion. 



A leaf is essentially an expanded surface, an organ adapted for 

 increasing the green surface of a plant, so as to expose to the li^ht 

 and air the greatest practicable amount of cellular tissue containing 

 the characteristic green matter known as " chlorophyll." This sub- 

 stance is situated in the loosely arranged cells in the interior of the 

 leaf, but not in those of the epidermis, which can easily be peeled off, 

 and are then seen to be colourless, containing nothing but air. The 

 green cells below the epidermis of the upper surface of the leaf are 

 more closely packed, and are oblong in outline, being placed endwise, 

 or with their long diameter at right angles to the surface of the leaf. 

 It is on this account that the upper surface of leaves is commonly 

 darker than the under surface, and the arrangement of the cells so as 

 to expose the least possible surface is designed as a protection for the 

 underlying cells from the too powerful evaporating action of direct 

 sunshine. This provision is even more complete in evergreens and 

 plants of arid climates, there being in such cases two layers of these 

 oblong cells very compactly arranged. The cells below these are 

 loose, with numerous and large interspaces, which contain air. 



