STRUCTURE OF LEAVES. 



145 



advancing point. This accounts for the wonderful facility with which it penetrates 

 the soil and finds its way uninjured into the hardest earth. 



727. Dictyogens. In those few Monocotyledons which bear reticulated leaves 

 (Smilax, Dioscorea), the Dictyogens of Dr. Lindley, the roots exhibit a structure re- 

 sembling that of exogenous stems. 



STRUCTURE OF LEAVES. 



728. Nature of the leaf. The leaf may 

 be regarded as an expansion of the two outer 

 integuments of the bark, or of the green bark 

 and the epidermis, expanded into a broad, 

 thin surface by a woody framework proceed- 

 ing from the medullary sheath and the liber. 



729. The framework of veins is therefore 

 fibro-vascular, abounding in spiral vessels, and 

 strengthened with liber. 



730. The parenchyma exists in two strata 

 more or less distinct. In all those leaves 

 which are ordinarily horizontal in position, 

 one surface being upward and the other down- 

 ward, these two layers are dissimilar ; but in 

 leaves with a vertical lamina (iris), and in 

 phyllodia (§ 307) the two layers are similar. 



731. The layers described. The superficial layer 

 of empty tabular cells, belongs to the epidermis. Next 

 beneath this, in the surface on which the sun shines, 

 are one or two layers of oblong cells placed perpendicularly to that surface, and 

 more compact than the cells beneath them, which are pervaded by intercellular 

 passages and by the veins. 



732. Place of the stomata. The stomata as a rule belong to the 

 shaded side of the leaf, avoiding the sun's direct rays. On the sunny 

 side there are few comparatively or none. In the submerged leaves 

 of water-plants the epidermal layer is hardly distinguishable, and is 

 wholly destitute of stomata. In such leaves as float upon water (water 

 lilies) stomata are found in the upper surface alone. 



733. The chlorophylle. Within all the vesicles of the paren- 

 chyma are seen adhering to the walls the green globules of chljro- 

 phylle, which give color to the leaf — dark green above, where it is more 

 compact, paler beneath, where the cells are more loose and separate. 



734. Vessels of cienchyma pervade the under-layer of paren- 

 chyma, returning the elaborated juices through the petiole into the cam- 

 bium layer. 



10 



603, Section of a stem p.t the ori- 

 gin of a leaf ; p, cellular, or pith ; 

 a, vascular, the medullary sheath 

 sending off a bundle into the 

 leaf-stalk; d, the swelling (pul- 

 vinus) just below the articula- 

 tion of the leaf-stalk (I) ; b, tha 

 axillary bud. 



