MORPHOLOGY OF HIGHER PLANTS. 211 



The petiole generally shows the structure of the midrib as far 

 as concerns the mestome-strands, but possesses furthermore a 

 more or less strongly developed parenchyma, the cells of which 

 are colorless, thin-walled and which may often be traced to the 

 leaf-blade itself, where it surrounds the stronger veins, causing 

 them to project as ribs and to be much thicker in cross-section 

 than the adjoining chlorenchyma. 



From a histological point of view the leaf structure of 

 Dicotyledons resembles very closely that of the Monocotyledons, 

 except that in the latter the palisade-cells often radiate towards 

 the center of the mestome strands. There are, however, 

 many instances of a similar development in the leaves of 

 Dicotyledons. 



Abnormal structures are common in leaves, especially in such 

 as are not held in a horizontal position, but vertical, as those of 

 Eucalyptus, the Irideae, etc. 



The Epidermis forms the surface of the leaf and may con- 

 sist of one or more layers of cells. The outer walls are cutinized, 

 and when nearly smooth the leaf is said to be glabrous. They 

 may be covered or whitened with a bloom, as in magnolia, when 

 the leaves are spoken of as glaucous. In other cases the outer 

 walls of the epidermal cells are modified to hairs (Figs. 283-285). 



INNER MORPHOLOGY OF THE FLOWER. 



The inner structure of the flower bears a close resemblance 

 to that of the stem and leaf. The bracts in almost all particulars 

 are like the foliage leaf of the same plant and the flower stalk 

 closely resembles the foliage stem. The calyx, while resembling 

 the foliage leaf, usually contains calcium oxalate in greater 

 amount, and the chlorenchyma consists wholly of rather loose 

 chlorophyll parenchyma ; the outer or under epidermis contains 

 the stomata, and if hairs are present, they also arise from this 

 surface ; the fibrovascular bundles are generally simple in struc- 

 ture, although in some cases, as in lavender, sclerenchymatous 

 fibers are strongly developed. 



