168 TRANSACTIONS OF THE [Sess. lxxxvii 



The leaf of this phmt exhibits a peculiar and unusual 

 structural feature. On the adaxial side, from the base of the 

 leaf to the tip, there runs a strip of tissue which, compared 

 with the rest of the leaf, is almost translucent. This area 

 we have named the " window " of the leaf. In transverse 

 section this translucent tissue is found to occupy about 

 four-fifths of the total area, and microscopic inspection shows 

 that it is structurally different from the remaining portion,, 

 or what may be called normal tissue of the leaf. 



In the normal tissue, a layer of wax is present on the external 

 surface, covering the epidermis, which consists of two rows of 

 cells. These are arranged regularly, are rectangular in shape 

 and thick- walled, the external walls of the cells of the outer 

 row being especially thick. The inner row of cells of this 

 double epidermis is interrupted at intervals by oil-ducts. 

 Within the line of ducts, the normal tissue of the leaf is com- 

 posed of parenchymatous, chlorophyll-containing cells, of 

 fairly uniform size and shape. Vascular bundles, varying in 

 number in different leaves, are embedded in the general 

 parenchyma of the leaf and are arranged somewhat irregularly. 

 Stomata are few in number, and where present are found in 

 pits. 



In the translucent tissue of the leaf a coating of wax is also 

 present, covering as before a two-layered epidermis. But no 

 oil-ducts are present and stomata are absent. The deeper- 

 lying tissue is parenchymatous, the cells larger than in the 

 normal tissue, with thinner walls and without chloro- 

 plasts. Nor are vascular strands present in this translucent 

 tissue. 



It thus becomes an interesting question as to whether the 

 " window " is of any particular use to the plant. The trans- 

 lucency suggests that it may allow light to penetrate more 

 readily to the chloropliyll-bearing tissues, thus facilitating 

 photosynthesis. As the cell-walls are thin, light will penetrate 

 them more easily, and as the cells are larger than those of the 

 normal tissue, theie are fewer walls to pass through. 



Since the photosynthetic process results in an increase 

 in weight, if it can be shown that the increase in a control 

 leaf is greater than the increase in a leaf with the " window " 

 blocked, both exposed to the same intensity of light for the 

 same length of time, there would be reason to believe that the 



