348 Mr Corry. On some points in the structure and [Ap. 30, 



is attained. The guard-cells now lie almost horizontally ; though 

 derived from the epidermal layer they lie completely at the level 

 of the hypoderma, but they may easily be distinguished in that 

 they alone of all the ejridermal and hypodermal cells contain 

 chlorophyll grains. A large intercellular space usually underlies 

 each stoma ; this is formed by the wall of a cell belonging to the 

 cortical ground tissue which immediately underlies the stoma, being 

 caught inwards towards the centre of the leaf owing to the pre- 

 sence of one or several of the so-called " intrusive ingrowths," to be 

 immediately referred to, in it iu that position. The arrangement 

 of the stomata is invariably in linear longitudinal rows, and the 

 long axes of the clefts lie in the same plane as the long axis of the 

 leaf while the guard-cells lie right and left of them : since they 

 occur on both sides of the leaf alike they are found in situations 

 which are exposed to light as well as those which are sheltered 

 from it, and are most numerous in the former. The type of forma- 

 tion exhibited by the stomata of Pinus belongs to the second great 

 type of Strasburger (loc. cit.) where several epidermal cells take 

 some part in the formation of a stoma, and to the second division 

 of that type, viz. that in which the divisions to form complemental 

 cells affect all the cells immediately surrounding the stomata- 

 mother-cells. It is most nearly related to the type of Salisburia 

 adiantifolia and then to Cycas revoluta and Araucaria imbricata. 



Except in the points just noticed the epidermal cells do not 

 present any marked peculiarity, their external walls are cuticula- 

 rized to an extremely large extent, and their walls are much 

 thickened and exhibit stratification and well-marked canals running 

 outward from the lumen, which is consequently very much 

 reduced and almost obliterated. The line of distinction between 

 the portion of the wall which is merely thickened but not chemi- 

 cally changed and that which is so altered is easily rendered evi- 

 dent on application of Chlor. Zinc. Iod. (Schultze's) solution. 



At each angle of the leaf in the transverse section an epidermal 

 cell which is very much larger than the rest projects, the external 

 convex wall of which is especially thickened. 



II. Periblem. 



* Ground Tissue or Mesophyll. 



A. Hypoderma. The cells of this when mature are very 

 thick-walled so as to be sclerenchymatous, exhibiting well-marked 

 middle lamella?, stratification, and canals running from the very 

 much reduced lumen to the exterior; they are not however coloured. 

 They are elongated in a direction parallel to the length of the leaf 

 and prosenchymatously arranged, forming fibres which contribute 

 to the greater firmness and elasticity of the epidermis which they 



