NUMBER OF LEAVES 



55 



119,000 leaves of an average-sized beech tree have a superficial 



leaf-area of 341 square yards. A fir tree with 39,080,000 leaves 



has a superficial leaf-area of 2180 square yards. A pine tree 



with 20,043,000 leaves has a superficial leaf-area of 940 square 



yards. The number of pine and fir needles is indeed amazing. 



A pine whose stem has a diameter of some 5 inches has 



millions of such leaves, and the fully grown firs and pines 



with a stem diameter of 20 



inches have from 10 to 20 



millions, whilst the largest 



specimens with a diameter 



of 30 to 35 inches will bear 



from 30 to 40 million needles. 



Some of the factors which are 



involved in transpiration are 



the number of leaves and their 



superficial area, the number 



of stomata, and the varying 



humidity of the surrounding 



air. 



These figures give some 

 slio'ht idea of the enormous 

 part that vegetation plays Fig. 17. Transverse section through the 

 in the humidity of the at- leaf of the Hellebore, showing, from 

 I T4- ^o 4-^ o lo^rro above downwards, the uppcr epiclermis, 



mosphere. It is to a large ^^^ p^Hsade cells, the spongy tissue (in 

 extent a fact that with the which a vascular bundle is seen), the 

 destruction of forests the hu- lower epidermis, in which is shown a 



■js. -p j-1 •« -p^Uo «..^ sinorle stoma opening into a large mter- 



midity of the air falls and cellular space. 

 tends ultimately to disap- 

 pear. Rain and fog and dew diminish, the soil becomes 

 dried up and vegetation ceases to exist. 



Apart from the tracheids, that bring the water with salts 

 in solution from the roots to the leaves where by osmosis it 

 enters the mesophyll cells, there is a system of vessels called 

 sieve-tubes which take the elaborated food from the leaf to 

 those parts of the plant which require it, either for their 

 growth or to store it up as reserve food after it has been 

 reconverted into starch. This acts as a reservoir or store of 

 food for next year's growth. 



