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PHYSIOLOGY OF THE LEAVES OF CERTAIN CONIFERS. 549 



number of ovoid cells more or less loosely aggregated and radia- 

 ting from the central fibro-vascular bundle. Their walls are 

 thickened, straight or very generally sinuous or undulated. Hy- 

 poderm-cells exist beneath the epiderm, especially at the angles of 

 the leaves. Stomates are found on all four sides, but most abun- 

 dantly on the two lower ones. The resin-canals are usually 

 subepidermal. 



In the flat-leaved Silvers there is generally a layer or two of 

 " palisade "-cells beneath the hypoderm of the upper surface. 

 These closely aggregated palisade-cells are filled with chlorophyll, 

 and have their long axes at right angles, or nearly so, to the 

 looser cells of the subjacent parenchyma. The hypoderm is on 

 both surfaces, in one or two layers, but always thickest at the 

 margins of the leaf and over the central fibro-vascular bundle. 

 The palisade-cells are usually confined to the upper surface 



Resin-Canals. — It forms no part of my present purpose to do 

 more than allude casually to the resin- canals, the differences in 

 the position of which have been used as points whereby to dis- 

 criminate species by Bertrand and M'Nab. They sometimes 

 vary in the same species according to stage of growth, age, posi- 

 tion, &c. I would only note here the unusual position of the 

 canals beneath the upper epidermis, or, at least, above the pali- 

 sade-cells, in Picea ajanensis, which may be described as a flattish- 

 leaved Spruce, and the existence of a solitary canal below the 



mowiczii, as in the section Tsuga. It is inter- 

 esting also to notice the fact that these resin-canals are often 

 enclosed within a sheath of hard hypoderm-cells, and that they 

 are often placed in grooves and thus protected from pressure by 

 the midrib and other prominent portions of the leaf. 



Movements of the Leaves. — Some of the Silver Firs are endowed 

 with a power of motion by means of which the leaves are raised or 



Max 



* In Abies (or Pseudotsuga) nobilis, a flat-leaved species, M'Nab notes the 

 almost complete absence of palisade-cells — a circumstance which he connects 

 with the presence of stomates on both surfaces of the leaf ; and in A. magnified, 

 an angular-leaved species with stomates on all sides, the palisade-cells are also 

 said to be wanting (M'Nab, l c. p. 701, t. 49. figs. 29, 30, a). JI'Nab's descrip- 

 tion, however, does not tally with his figures; for in ^g. 29a palisade-cells are 

 well developed, as also in fig. 30 (magnified), in which latter numerous sto- 

 mates are shown on all four sides. Bertrand says that in Pseudotsuga, to which 

 section the two species just named belong, " Le tissu fondamental est diffe- 

 rencie en parenchyme en palissade et en parenchyme rameux." As there is 



