548 DR. M. T. MASTERS OK THE MORPHOLOGY AtfD 



between the projecting pulvini, where two or three layers of 

 hypoderm-cells may be found. The whole is encircled by thick- 

 walled epidermal cells. 



The pulvini, then, are mere outgrowths from the suberous 

 layers of the cortex, and do not form a part of the leaf* 



In the Silver Firs the pulvini are scarcely at all developed. 



The Leaves. — The leaves of the Spruces are usually more or 

 less four-sided and needle-like, given off on all sides of the shoots, 

 ascending, but not appressed. They are more or less twisted at 

 the base, so as to bring them into the most advantageous position, 

 and to prevent them from encroaching on their neighbours. 



In the Silver Firs the leaves on the lateral branches are given 

 off on all sides ; but those on the under surface more especially 

 are twisted at the base so as to bring them into nearly the same 

 horizontal plane as the upper ones. The uppermost leaves on the 

 side branches are either parallel in direction with the lower ones 

 (that is, at an angle with the axis of the shoot), or they are placed 

 at right angles with them (that is, parallel indirection with the 

 long axis of the shoot). In the former case the upper leaves are 

 shorter than the lower, so as only partially to overlap them ; in 



the latter case the leaves are of about equal length, and do not 

 overlap or interfere one with another. 



In form, the leaves of the Silvers are flattish, with an upper 

 and a lower surface. On the leader shoot of Abies JVordmanniana 

 and allied species the leaves vary in position according to age, 

 season of growth, or exposure. Sometimes they are closely ap- 

 pressed against the stem ; sometimes they spread horizontally ; 

 w r hile at other times, though " ascending," they are not closely 

 appressed to the stem, but are so placed that one edge is di- 

 rected towards the stem, the surfaces looking laterally. In this 

 latter case, while provision is made for exposure of the surfaces, 

 provision is also made to prevent, as far as practicable, the over- 

 shadowing of one leaf by another. In A. nobilis the leaves on 

 the leader shoot are appressed to the stem, each leaf ascends ob- 

 liquely, so that successive leaves form a conspicuous spiral, the 

 tip of one leaf touching the base of the one next above it. 



Internal Structure. — But few words will be necessary under 

 this heading, as the publications of Bertrand and M'Nab contain 

 the details requisite for my present purpose. It may suffice to 

 say here that in the angular- leaved Spruces there are usually no 

 palisade-cells. The ground-tissue or parenchyma consists of a 



