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ANATOMY, AND L1FE-HISTOBY OP THE CONIFERS. 



251 



rated by the stomata, thickened by additional layers in certain 

 spots, as at the angles of the leaves, over the midrib, round the 

 resin-canals, and sometimes broken up into thick detached islets, 

 a remarkable instance of which is afforded in Pinus Coulteri, the 

 leaves of which have triangular wedge-shaped masses of hypoderm 

 projecting into the mesophyll. Stahl says that in Abies pectinata 

 the hypoderm-cells are most abundant on the side of the leaf ex- 

 posed to the sun, least so on the shady side. 



Messrs. Coulter aud Rose point out the presence in certain of 

 the North-American species of Pinus of a layer of relatively thin- 

 walled cells between the epidermis and the strengthening-cells 

 proper. This layer is indicated by them in the following species: 



P. 



contorta 



Ibicaulis^flea 

 cata, Pngeln 



Montezuma 



efli 



The Mesophyll. — Within the hypoderm, and forming the chief 

 substance of the leaf, is the mesophyll, generally wholly cellular, 

 the constituent cells containing chlorophyll being uniform or 

 nearly so, as in Picea. while in others there is a distinction into 



palisade-cells, on 



(Abies), or on both surfaces (A 



wibricata), but in an} r case most conspicuously on the side most 



exposed to the light, and branching cells with lacunae between 

 them (Abies). In some species the cells appear to radiate from 

 the central bundle (Pi 

 for the involution 



, spp.). In Pinus they are remarkable 



s of their walls, causing flat projections into the 

 interior of their cavities. In other cases a series of colourless 

 cells spread laterally from the central bundle in the midst of the 

 chlorophyll-containing cells of the mesophyll and constituting the 

 transfusion- tissue. This may be seen in Keteleeria Fortune* I, 

 Pseudo-larix Eampferi !, some species of Podocarpus, e. g. P. 

 cUlensis ! Very large, branching, thick-walled cells like those of 

 welwiUchia are found in Araucaria imbricata and other species of 

 the order. 



The £ esin-canah. — Traversing the mesophyll are the resin- 

 canals, which demand notice from the constancy of their arrange- 

 ment • Their presence is, to some extent, variable ; but when 

 they do occur it is nsnallv in ZUfinifA numbers and in certain 





l 





In the stem they occur in the middle of the cortex, generally as a single 

 ^ but k 8 ome species of Pinus where the cortex is very thick there are several 



s. They also occur iu some cases in the secondary wood, 1 

 *ey are to be found in the pith. 



hils 





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