THE LEAF 121 



its zone of growth becomes limited to tlie basal portion. The 

 apex therefore represents the oldest portion of a leaf; only in 

 the fronds of ferns growth continues to take place at the apex, 

 and sometimes for several years {e.g., in Gkichenia). If a leaf 

 is to become pinnate, lateral projections begin to make their 

 appearance on the young leaf, beginning at the base and pro- 

 gressing towards the apex. 



The further development of the leaf differs according to the 

 species and the locality, the only principle of construction 

 observed in all cases being, as we have already seen, the effort 

 to produce a great expansion of the assimilating tissue. 



If this assimilating tissue is in any danger of being dried 

 up or damaged by too intense an illumination, nature protects 

 it by various contrivances. Thus the Helichrysce of the Cape 

 are covered with a tremendous development of felted hairs ; 

 the epidermal cells of Agave americana have an enormously 

 thickened cuticle, still further protected by a layer of wax ; 

 while Ficus elastica has, as we have seen, a triple epidermis, 

 one layer of which has the function of storing water. In 

 some cases the leaf is reduced to the form of a needle (Mela- 

 leuca, Metrosideros) ; in other cases the leaf assumes a vertical 

 position, so that the sun illuminates the leaf edgeways (Euca- 

 lyptus). In this case a pallisade parenchyma is formed on both 

 sides of the leaf. Nature can also reduce the number of the 

 stomata according to circumstances. In many horizontal and 

 tough leaves the shining upper surface of the leaf is devoid 

 of stomata ; in other cases they are placed in protected pits. 

 Floating leaves (Nym/phaa) have stomata only on their upper 

 surface ; the leaves of plants which always have a large supply 

 of water have very large guard cells, &c. 



Whatever may be the arrangement of the assimilatory 

 tissues within the leaf, it always makes use of the first rays of 

 light which it absorbs to develop its real working organs, the 

 chlorophyll granules. The rudiments of these are present in 

 the protoplasm of every newly-formed cell, and have entered it 

 from the mother cell in the form of a small protoplasmic struc- 

 ture,^ which under the influence of the light begins to form the 

 green chlorophyll, and causes the young leaf to become green. 



' Trophoplast. 



