PTERIDOPHYTES. Ill 



walls are marked with shallow depressions, whose bottoms are finely 

 pitted. These are the so-called " sieve tubes." 



For microscopical examination, either fresh or alcoholic material may 

 be used, the sections being mounted in water. Potash will be found use- 

 ful in rendering opaque sections transparent. 



The leaves, when young, are coiled up (Fig. 67, (7), owing to 

 growth in the earlier stages being greater on the lower than on 

 the upper side. As the leaf unfolds, the stalk straightens, and 

 the upper portion (blade) becomes flat. 



The general structure of the leaf stalk may be understood 

 by making a series of cross-sections at different heights, and 

 examining them with a hand lens. The arrangement is essen- 

 tially the same as in the stem. The epidermis and immediately 

 underlying ground tissue are dark-colored, but the inner ground 

 tissue is light-colored, and much softer than the corresponding 

 part of the stem ; and some of the outer cells show a greenish 

 color, due to the presence of chlorophyll. 



The section of the fibro-vascular bundle differs at different 

 heights. Near the base of the stalk (Fig. D i) it is horseshoe- 

 shaped; but, if examined higher up, it is found to divide 

 (n, in), one part going to each of the main branches of the 

 leaf. These secondary bundles divide further, forming the 

 veins of the leaflets. 



The leaflets (J5J, F) are one-sided, the principal vein running 

 close to the lower edge, and the others branching from it, and 

 forking as they approach the upper margin, which is deeply 

 lobed, the lobes being again divided into teeth. The leaflets 

 are very thin and delicate, with extremely smooth surface, 

 which sheds water perfectly. If the plant is a large one, some 

 of the leaves will probably bear spores. The spore-bearing 

 leaves are at once distinguished by having the middle of each 

 lobe of the leaflets folded over upon the lower side (F). On 

 lifting one of these flaps, numerous little rounded bodies 

 (spore cases) are seen, whitish when young, but becoming 

 brown as they ripen. If a leaf with ripe spore cases is placed 



