BY A. G. HAMILTON. 155 



than in the ordinary leaf, and the vestibules are shallow and 

 wider. The palisade tissue is not so close, and is composed of 

 shorter and narrower cells. The cuticle has not tlie l)rown tint of 

 that in the ordinary leaf, being- quite clear and transparent. The 

 capitate liairs occur very sparsely. The sclero])lasts are also rare. 

 There is little difference in the external characters, except that tlie 

 leaves of shade-plants have much darker green leaves, which may 

 be accounted for by the thinness of the cuticle, and the absence of 

 brown colour from it. 



There can be no doubt but that the thick cuticle is a powerful 

 factor in checking- transpiration and this is aided by the deeply 

 sunken stomates. The closeness of the palisade cells and the 

 absence of intercellular spaces also aid in this. The young leaf is 

 effectively protected by the thick coating of hairs. 



The most remarkable feature of the leaf is the great develop- 

 ment of scleroblasts. In the young leaf, even up to the time when 

 it has reached full size, there are none to be found; and we have 

 seen that, in tliose growing in the shade, they are rare or absent. 

 But as the mature leaf, exposed to sun and wind, grows older, 

 small masses of sclerenchyma appear in the middle region of the 

 leaf; these increase in size and number till they occupy a very 

 large part of the micl-region of the leaf — as much, I estimate, as 

 45 % of it. They begin their growth among the palisade cells, 

 and extend gradually outwards till they reach the epidermis, and 

 then the part touching the epidermis extends laterally. As to their 

 origin, it is most likely, as Sachs conjectures, that they are nothing 

 more than peculiarly developed parenchyma calls(l,p.l46). From 

 the fact that they develop most in leaves exposed to intense light, 

 heat, and wind, it would appear that their occurrence is a direct 

 consequence of those conditions. Their function is probably the 

 same as that of the closeness of the mesophyll, viz., diminishing 

 transpiration by, as Sachs states (l,p.l44), slowing the exchange 

 of sap between the parenchymatous tissue and the veins. 



The function of the sclerenchymatous fibres round the vascular 

 bundles is apparently to give rigidity to the leaf, but they would 

 also undoubtedly assist in slowing transpiration, as they are always 



