PHYSIOLOGY OF THE LEAVES OP CEETAIK CONIFEES. 551 



If we now correlate these facts, their significance becomes app a 

 rent. In the Silvers, where the leaves are so crowded as to 

 overlap each other closely, and w r here their relatively flat sur- 

 faces permit only one face at a time to be exposed to the light, 

 not only are the leaves twisted so as to bring them all into nearly 

 the same plane, but they are in many cases, if not in all, endowed 

 with a power of alternate elevation and depression, so that the 

 lower surface may be exposed to the light. Mutual interference 

 is also obviated by the smaller size of the upper leaves, or by the 

 different direction assumed by the upper and lower leaves respec- 

 tively. 



In Spruce Firs, as a rule, the leaves are less densely packed 



than in the case of the Silvers, and they are usually more or 

 less 4-sided. There is torsion at the base of the leaf, but, so far 

 as I have seen, little or no motion of elevation and depression*. 



Such movements seem not to be required in their case, as the 

 relative position and arrangement, as well as the internal struc- 

 ture, of the leaves are such as to secure a nearly equal amount of 

 exposure on all surfaces ; and there would therefore appear to be 

 the less reason for the presence of palisade cells on or just 

 beneath the surface most favourably placed as regards exposure 

 to light. The palisade cells appear to be especially concerned in 

 the process of nutrition and assimilation, for w T hich exposure to 

 direct light is especially advantageous. Where the shape of the 

 leaf permits such an aggregation, the cells in question are formed 

 in that position where they can best fulfil their work. "Where 

 the form of the leaf, on the other hand, is such that no one sur- 

 face is particularly favoured as regards exposure to the light, 

 there is no such marked difference between the form and the func- 

 tions of the constituent cells. In this connexion it is of interest 



* The leaves of P. sitchensis, which are angular, have, however, a marked 

 power of movement, but they are destitute of palisade cells. P. ajanensis, 

 another Spruce, is remarkable for the movements of its leaves, which, however, 

 are flat and generally, but not always, possess palisade cells; moreover the 

 resin -canals are beneath the upper epidermis, above the palisade cells — a very 

 unusual position. These exceptional cases — and doubtless there are others — 

 do not invalidate the general rule that palisade cells accompany flat leaves 

 which are mobile, and are not found in the more distinctly angular ones, which 

 are nearly if not quite motionless. 



LINN. JOUEN. BOTANT, VOL. XVII. 2 S 



