PHYSIOLOGICAL STRUCTURE. 



97 



a. The tissue of the lamina, iii hke manner, must essentially 

 correspond with the outer integuments of the bark, of which it 

 is but an extension. That peculiar form of cellular tissue of 

 which it is composed is called parenchyma. 



261. The parenchyma of the leaf exists in two layers, as 

 might be inferred from the manner in which it is produced 

 (221). In all those leaves which are ordinarily horizontal in 

 position, one surface being upwards and the other downwards, 

 these two layers are dissimilar in structure ; but m those leaves 

 where the lamina is vertical, as in the iris, they do not mate- 

 rially differ. 



a. The whole structure is, of course, clothed with the epi- 

 dermis. 



262. The internal structure of the p.ai-enchjTna is more complicated than would 

 be at first supposed. A powerful microscope is necessary for its examination. 

 Let an exceedingly thin ■paring be taken from a vertical section of the lamina 

 and submitted to the solar (or compound) microscope, in such a manner that the 

 rays shall pass from section to section. Fig. 36 represents a magnified view of 

 such a paring of the leaf of the lily, which may be regarded as characteristic of 

 leaves in general. 



FIG. 36. — 1, Magnified section of a leaf of the li!j' ; 2, of the epiJermis with sloraata. 



263. The upper surface (a, o) is thus seen to consist of tlie flattened cells of tb.e 

 epidermis, an-ang^d in a single layer. Just beneath tliis (6, 6) is the more com- 

 pact part of the parenchyma, consisting of a layer of oblong cells placed in such a 

 position that their longer a.xis is perpendicular to the leaf's surface. Next below 

 we meet wth the parenchyma of the lower surface (c, c), composed of oblong 

 cells arranged longitudinally, and so loosely compacted as to leave larger empty 

 spaces between. Lastly, we find again the epidermis {d^d) of the under surface 

 with stomata (f, e), opening into air-chambers. 



a. Within all the vesicles of the parenchyma are seen adhering to the walls, 

 the green globules (chlorophyll) which give color to the parenchyma, — dark 

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