310 THE FOLIAGE LEAF 



mesophyll is palisade, thus reducing the rate of transpira- 

 tion. Leaves held on the plant in a position in which 

 both sides are equally illuminated frequently have the 

 two surfaces alike, palisade being developed under each 

 epidermis. Inversely, in plants growing in damp and 

 shady situations the palisade tissue takes on the 

 characters of spongy tissue, the cells are short and are 

 separated by large intercellular spaces ; and in extreme 

 " shade leaves " the whole mesophyll is of the spongy 

 type. The deeper the shade and the damper air 

 (conditions generally found together in nature) the 

 greater the total volume of intercellular spaces in 

 proportion to the total volume of the mesophyll cells. 



Water Tissue. The epidermis of a typical foliage leaf 

 acts, as we have already remarked (p. 275), as a water 

 store for the mesophyll cells ; and, as we have also seen 

 (p. 276), some leaves have a many layered epidermis 

 which greatly increases this water store. In extreme 

 cases the epidermal water tissue may form the greater 

 part of the thickness of the leaf. Part of the mesophyll 

 may also be very poor in, or even destitute of, chloro- 

 plasts, e.g. the central tissue of isolateral leaves, and this 

 also acts as a water store for the photosynthetic tissue. 



Vascular Bundles (Veins). Water and salts are brought 

 to the mesophyll of the leaf through the vessels and 

 tracheids of the xylem ; while sugars and organic 

 nitrogenous substances are conducted away from the 

 mesophyll through the sieve tubes of the phloem. A 

 strand of xylem and a strand of phloem are associated 

 together to form a vascular bundle, the xylem being 

 towards the upper, the phloem towards the lower face 

 of the leaf. In the leaves of dicotyledons (the greater 

 number of seed plants) there is a large central vein, 

 the midrib, running up the centre of the leaf, and this 



