TRANSACTIONS OF SECTION K. 909 



is completely absent from the woods of this series ; and this fact, and also the 

 different nature of the ground vegetation, distinguishes them from the various 

 types of Lowland Oak Woods. The woods are degenerating, and numerous 

 derivative scrubs occur. The whole of this series falls into the general Oak-Birch- 

 Heath type, and the slight modifications are due to factors correlated with 

 altitude. 



The above Oak and Birch series of woods is developed on siliceous soils alone: 

 on the soils of the carboniferous limestones the natural Avoods belong to the 

 type dominated by the common Ash (Fraxinus excelsior). Quercus sessilijlora is 

 completely absent from and Q. pedunculata is quite local in the Ash Woods of 

 Derbyshire. The Ash Woods have the same altitudinal range as the Q. sessili- 

 Jlora Woods : above 1,000 feet, scrubs derived from pre-existing Ash Woods 

 occur. 



Certain intermediate types, such as Oak-Ash Woods and Ash-Birch Woods, 

 were mentioned ; and the ground vegetation of the main types was described. 

 The present altitudinal range of the upland woods was compared with that of 

 former times. 



MONDAY, SEPTEMBER 7. 



Joint Discussion ivith Section D on the Determination of Sex. 



See p. 738. 



The following Papers and Reports were then read : — ■ 



1. The Optical Behaviour of the Epidermal Cells of Leaves. 

 By Harold Wager, F.R.S. 



The observations of Haberlandt ^ and others have shown that the structure 

 of the epidermal cells of the leaves of many plants is such that the rays of light 

 which fall upon them are refracted and brought to a focus either within the 

 epidermal cells or on their basal walls, or in the leaf among tlie chlorophyll- 

 containing cells. Haberlandt appears to consider that this function is restricted 

 to the cells of the upper epidermis, but although it is in most cases not so 

 marked, it is equally common to the cells of the lower epidermis. 



In some leaves the epidermal cells are so regular in outline that they may 

 be compared to the corneal facets of the compound eyes of insects ; but usually 

 they are not so regular, and often possess a wavy outline. Nevertheless they 

 are all capable of bringing the rays of light which fall upon them to a more or 

 less definite focus. These lens cells are very widely distributed, and it is very 

 rare to find a plant which does not possess them in some degree, although, as 

 Haberlandt has pointed out, they are more highly developed in shade plants 

 than in those exposed more fully to the light. From some observations which 

 I have recently made, they would also appear to be more highly developed in 

 the shade leaves of some plants than in the leaves of the same plant exposed to 

 the light. 



Cells with a very regular outline sucb as occur in species of T radescantia on 

 both sides of the leaf, the upper epidermal cells ot' Li</usfrum ovalifolium, 

 various Orchidacea, &c., act as very efficient lenses, and not only bring the light 

 to a focus, but are capable of forming clear and distinct images of objects near 

 them. In one case Haberlandt was able to obtain a somewhat indistinct photo- 

 graph of a microscope stand which was focused upon the basal wall of the 

 cell. More recently I have been able, by appropriate manipulation with the 

 microscope and the use of the Gordon photo-micrographic apparatus, made by 

 Messrs. Beck, to obtain photographs of a variety of objects through cells both 



' Lichtsinnesorganc der Zaubblatter, 1905. 



