PROCEEDINGS OF SECTION D. 485 



The purpose of this thick coating is evidently protection of the 

 tender leaf against dry conditions. The 3'oung leaf, when about 

 15mm. in length, shows no palisade or spongy tissue : it consists of un- 

 differentiated cells, except that one layer of epidermis is developed and 

 the inner layer just indicated. The vessels, except the midrib, are not 

 formed, and there is no sign of the crypts and stomates. 



When the palisade and spongy layers are fully developed, and the 

 crypts and stomates complete, the thick-walled hairs drop off. 



The Mature Leaf. — The upper epidermis consists of two, or some- 

 times three, layers. The outer wall or cuticle is extremely thick. In 

 the just matured leaves there are oxalate of lime crystals in the cells 

 of the outer row, and the cells of the inner row are filled with protoplasm. 

 Later on the crystals and protoplasm disappear. The epidermis on the 

 lower face of the leaf is in a single layer, the cells being thick walled, 

 and usually filled with a tannin giving a blue-black reaction with iron 

 salts. 



The veins, instead of being round in section, as in ordinary leaves, 

 extend from the upper to the lower epidermis in flat walls, like bulk- 

 heads, and divide the mesophyll into small compartments. They are 

 characterised by a great development of the sclerenchyma. The 

 vascular bundles run along at the junction of palisade and spongy 

 tissues, and the sclerenchyma extends in a flat plate composed of two 

 rows of fibres from the upper epidermis to the vascular bundle, and 

 again below the latter to the lower epidermis. The thickening of the 

 walls is very marked, the lumen being almost obliterated. 



The vascular bundle proper is remarkable for the number of the 

 pitted vessels, which are recognised as the water-carrying element. The 

 predominance of sclerenchymatous tissue gives great strength and 

 firmness to the leaf. In addition, as already mentioned, the wall-like 

 shape of the bundles effectually partitions off the mesophyll into 

 separate chambers. 



Each of these compartments has in the centre a balloon-shaped cavity 

 — the crypt (Plate I., fig. 4). This is bounded on the top and on the sides 

 down to the widest part with palisade cells. Below this is the spongy 

 tissue, which consists of a network of slender cells with wide inter- 

 cellular spaces. Kound the neck and mouth of the crypt there is more 

 of the palisade tissue, shorter in the individual cells than in the upper 

 layer. The chlorophyll granules of the mesophyll are large, and there 

 are usually large starch grains present also. The palisade cells give a 

 strong tannin reaction with iron salts, while the spongy tissue shows 

 only slight coloration. 



The crypt itself is lined with large broadly elliptical stomates (Plate 

 I., fig. 5), which color very deeply with any of the protoplasmic stains. 

 From the spaces between the stomates arise very peculiar hairs. These 

 are club-shaped and thick walled, and have long curly cilia growing 

 from the tip (Plate I., fig. 6). They generally contain cell sap which 

 colors very faintly with the majority of stains. 



The whole structure of the leaf is admirably adapted to lessen 

 transpiration in dry weather. The thick cuticle on the upper side is 



