DIV. I 



MORPHOLOGY 



113 



than in the " sun-leaves." According to NORDHAUSEN'S investigations ( e2 ), however, 

 no direct influence of the illumination exists. There are also plants (e.g. Lactuca 

 scariola] which only form palisade cells in strongly illuminated leaves. 



In some plants layers of cells placed parallel to the surface instead of at right 

 angles to it are found in the usual situation of the palisade tissue. In the leaves 

 of the Pine and some other plants the same position is occupied by large, more or 

 less isodiametric cells the internal surface of which is considerably increased by 

 foldings of the cell walls. 



Below the palisade parenchyma comes what is known as the 

 SPONGY PARENCHYMA (sp), which extends to the lower epidermis (ep"). 

 The spongy parenchyma consists of irregularly -shaped cells with 

 wide intercellular spaces and less chlorophyll than in the^ palisade 



7^ 



FIG. 130. Transverse section of a leaf of Fagus sylvatica. ep, Epidermis of upper surface ; ep", 

 epidermis of under surface ; ep'", elongated epidermal cell above a vascular bundle ; pi, palisade 

 parenchyma ; s, collecting cells ; , sp, spongy parenchyma ; A% idioblasts with crystals, in f 

 with crystal aggregate ; st , stoma. ( x 360. After STRASBURGER.) 



tissue. The wide intercellular spaces stand in immediate relation to 

 the stomata of the lower epidermis and serve for the transport of gases 

 to the palisade cells. 



HABERLANDT has estimated the number of chloroplasts per square millimetre 

 of a leaf of Ricinus to be 403,200 in the palisade parenchyma and 92,000 in the 

 spongy parenchyma. Thus in this case 82 per cent of the chloroplasts would belong 

 to the upper and only 18 per cent to the lower side. 



Colourless WATER -STORAGE TISSUE is frequently present in the mesophyll 

 (Fig. 129 W}. It may be surrounded by the assimilatory tissue or be situated 

 externally to this below the epidermis. In the latter case the water-storage tissue 

 usually consists of the more internal cells of a many-layered epidermis. 



EPITHEMA and WATER -STOMATA C 53 ). The mesophyll of the Jeaf- blade in 

 certain families of Monocotyledons and Dicotyledons forms peculiar structures 

 between the swollen ends of vascular bundles and the epidermis. They are com- 

 posed of small living cells with colourless cell sap. the intercellular spaces being 

 filled with water. These masses of tissue go by the name of EPITHEMA and bring 

 about the excretion of drops of liquid water. In this process their function is 



