386 MANUAL OF BOTANY 



hypopodium, mesopodium, and epipodium become separately 

 recognisable. The growth is for a time apical, but this soon 

 ceases, except in Ferns, and the further development is caused 

 by basal or intercalary growth. 



The structure of the axis of the leaf differs materially 

 from that of the winged outgrowths from it. In the epipodium 

 the wings are usually much greater in extent than the axis 

 which bears them, so that the structure of the latter is soon 

 lost, and the leaf-blade when seen in section comes to appear 

 altogether different from the rest of the phyllopodiiim. 



The Meso])odium or Petiole. — The axis of the phyllopodium 

 is most readily examined in the intermediate region, or petiole, 

 which seldom becomes winged. A section in this region has a 

 somewhat flattened or slightly concave upper surface and a 



rio. 771. 



Fig. 771. Sections of two types of phyllopodium. In A the bundles are 

 Eirrauged separately, eac-li having its own endoderniis, ed., and pericycle, jt. 

 In E tliey are arranged in a ring in tlie mex'istele witli a common endo- 

 dcrmis and pcric_\c*]c. Alter Van Tiegliem. 



larger convex lower face (fig. 772). The same regions can be 

 distinguished as in the stem, viz. an epidermis, a cortex, and a 

 certain amount of stelar tissue. 



The epidermis is similar to that of the stem : its cells are 

 frequently prolonged into hairs ; generally stoiiiata are present. 

 The cortex is usually composed of elongated parenchymatous 

 cells with numerotis intercellular spaces. Chlorophyll grains 

 are present in the external layers. The tissue is often found to 

 contain liypoderinal layers of collenchyma or solerenchyma. 

 which may be continuous with similar layers in the stem or 

 may be peculiar to the petiole. 



In a few leaves, e.g. those of Hoya carnosa, the petiole 

 contains a layer of cork throughout its length, a little below the 

 epidermis. 



