92 ELEMENTS OF PLANT ANATOMY. 
phytes, there are found three distinct systems, epidermal, vas- 
cular, and mesophyll, the latter corresponding to the ground 
system of the stem. As has been seen from its method of de- 
velopment the leaf does not possess any urmeristem cells, there- 
fore never ends in a bud. | 
In the Pteridophytes a sharply distinct epidermis covers both 
sides of the leaf. The inner portion consists of roundish or 
parenchymatic, chlorophyll-holding cells, with frequent and 
large air-spaces between, and vascular bundles disposed so as to 
give support and conduct material to the chlorophyll-holding 
mesophyll cells. ; 
In the phanerogams the structure is still more complex, the 
mesophyll cells being usually separated into two distinct classes 
of tissues each consisting of one or more layers. These tissues 
Fia. 41. 
Cross-section through leaf of Cyclamen Europaeum. O over, O' under epidermis. S stoma. 
Mmesophyll. P palisade tissue. S spongy tissue with vascular bundle, g. x 300.— 
(Wiesner.) 
are called palisade and spongy tissue. The former occurs 
usually immediately below the upper epidermis and consists of 
one or more layers of oblong cells, so situated that their long 
diameter is at right angles to the surface of the leaf. Inter- 
cellular spaces are very seldom found between the cells of this 
