I024 



A TEXTBOOK OF THEORETICAL BOTANY 





Pinus is uncertain, but physiologically the effect of the flanges in increasing 

 the internal surface of the cell is identical, and hence the surface available 

 for the exposure of chloroplasts is also increased. Such cells are not confined 

 to the family mentioned above and they occur sporadically in many families, 

 both of Dicotyledons and Monocotyledons (Fig. 1013). 



The lacunar tissue plays a subsidiary part in photosynthesis, and it is 

 uncertain whether its cell walls are sufficiently permeable to allow for much 

 gaseous interchange or for the evaporation of water. Lewis has recently 

 shown that the walls of the spongy tissue cells are highly hydrophobic and 

 unwettable by water. They show no tendency to absorb water which is 

 injected into the leaf, though it is taken up by the cells of the veins. These 



Fig. 1013. — Triticitm vidi^are. Longitudinal section of leaf showing 

 lobed mesophyll cells. {After Briiner diid Weaver.) 



observations have not been extended to the palisade cells, and it may prove 

 that there is a differentiation in this respect between the two layers and that 

 the palisade is the chief seat of transpiration as w^ell as of photosynthesis. 



The external cuticle of the leaf is often continued inwards through the 

 stomatal pore and extends as a relatively delicate film over some part of the 

 spongy mesophyll. In some cases there is evidence that it may extend over 

 the whole surface of that tissue, which may account for its unwettable 

 character. 



Injury to the mesophyll may induce the growth of tyloses (Fig. 10 14) 

 from the spongy cells into the intercellular spaces, blocking them in the same 

 way that the cavities of old vessels become blocked. Mesophyll cells exposed 

 by injury may also form a phellogen and the wounded surface is thereby 

 covered and protected by the development of wound cork (Fig. 1015). 



The lacunar tissue clearly serves at least two functions ; the intercellular 

 spaces provide a reservoir of air and water vapour which may be of great 

 benefit to the palisade cells as a buffer between them and external conditions, 



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