4 A HANDBOOK OF CONIFERS 



shoots which develop from the buds vary greatly in colour and 

 degree of hairiness ; they also furnish in some instances a reliable 

 means of identification. In Larix, Pseudolarix and Cedrus two 

 kinds of shoots are produced, the one long and slender, with 

 scattered leaves, the other short and thick, with the leaves in 

 tufts at the extremities. The former are extension or leader 

 shoots, the latter resemble the spurs of fruit trees. 



Foliage. 



The leaves of conifers, which, except in a few genera, are ever- 

 green, vary considerably in appearance. The adult leaves are 

 ovate, needle-like, flat, and Unear, or assume the form of closely 

 pressed scales. In many cases the leaves which succeed the 

 cotyledons differ in form, attachment, or arrangement, and to 

 some extent in structure from those which characterize the adult 

 state of the tree. The arrangement of the leaves often varies on 

 different parts of the tree. Thus on the erect leading shoots of 

 various species of Abies, Picea, Pseiidotsuga, Tsuga, Taxus, and 

 Cephalotaxus, the leaves spread on all sides. On coning shoots 

 the leaves are vertically placed, while on the lateral branches 

 they are arranged nearly in one horizontal plane. The surface 

 of the leaf is perforated with stomata which are breathing pores 

 or apertures in the epidermis surrounded by guard cells leading 

 into an intercellular space below. In Pinus, Picea, Sind Abies the 

 stomata are disposed in longitudinal bands on one or both surfaces, 

 their position being indicated by white dots of glaucous bloom. 



Beneath the epidermis are usually found one or more layers 

 of long, thick- walled strengthening cells known as the hypoderm, 

 often forming an unbroken sheet or perforated by the stomata, 

 and sometimes thickened by additional layers. In the genus 

 Pinus the hypoderm varies considerably in the different species 

 and occasionally affords distinctive characters ^ which are men- 

 tioned in the description of the species. 



Traversing the mesophyll or spongy tissue of the leaf are the 

 resin ducts, which although variable in position usually occur 

 in definite numbers and in well-defined situations according to 

 genera and species (see Tsuga, Abies, and Pinus). These resin 

 canals are not very constant in their occurrence and are sometimes 

 absent. When present the canals terminate at the base of the leaf 

 and do not join on to those of the stem. In the centre of the 

 mesophyll is the bundle sheath or endoderm, consisting of a 

 single row of oval cells. These encircle the pericycle (thin- 

 walled) cells, in the centre of which is the fibro-vascular bundle 

 which constitutes the midrib of the leaf. In some cases (as in 

 Araucaria) there are several such bundles running through the 

 leaf. 



^ Shaw, Oenus Pinus, 6 (1914). 



