68 THE PLANT WORLD. 



below the middle to a rounded tip. (See Fig, 1.) On the leader and 

 on stronger shoots near the tree-top, the apices of the leaves are acute. 

 They persist for seven or eight years. Their color is dark green or 

 glaucous green. It is a fact worthy of notice, that this tree varies 

 quite markedly in color. I have observed in western Oregon two 

 trees standing fifty yards apart showing extremes in difference, the 

 one being; distinctlv blue as a result of the glaucous coverino- of the 

 leaves, while the other is as distinctly dark green with no suggestion 

 of the glaucous character. This latter character is therefore not con- 

 fined to Colorado and the mountains of Mexico as has been thought. 

 In these regions however, the glaucous covering may be more pro- 

 nounced. Figure 2 is a transverse section. The epidermis is made up 

 of small cells with thick walls (Fig. 3.) and a heavy cuticle which is 

 thicker on the upper than on the lower surface of the leaf. The 

 epidermal cells of the under side appear papillate in transverse section. 

 As the cells are several times longer than broad, the under surface 

 is thrown up into minute ridges. The lower surface only is penetrated 

 by stomata (Fig. 2st.) and is distinctly glaucous. Beneath the 

 epidermis lies a layer of long cells with thick walls which constitute in 

 part the mechanical tissue (Fig. 2s.) of the leaf. The layer is not 

 continuous. There is a strip along the middle of the upper side, where 

 it is further strengthened by a narrow second layer. On either side of 

 this median strip it is broken up or entirely absent. Around the 

 margin of the leaf as far as the resin ducts it is again continuous, while 

 it is absent from the under side, or is represented at least by a very 

 few scattered cells. 



The chlorophyll-bearing cells or parench3'ma of the leaf is easily 

 seen to consist of cells of two shapes. Those beneath the upper sur- 

 face of the leaf arc elonjjated at ricjht anojles to that surface, and con- 

 es o o ' 



stitute the palisade tissue. Between the palisade tissue (Fig 2/>.) and 



the under epidermis the parenchyma cells are irregular, with short 



branches, the ends of which articulate with each other so as to make 

 up a coarse network of cells with a continuous branching system of in- 

 tercellular spaces which permit the movement of gases and water 

 vapor. These spaces are in communication with hemispherical chambers 

 which are found beneath the stomata. Traversing the leaf from end 

 to end are two resin ducts, (Fig. 2d.) one of which lies near the 

 lateral angle of the leaf and against the lower epidermis, a position 

 which is more characteristic of the genus Abies — the true firs. 



{To he continued.) 



