DEAF OF RUTA. 193 



These cells are devoid of chlorophyll, swollen into a barrel shape, 

 and appear as if suspended between the green cells. At the 

 edges of the leaflets the outer sides of the epidermal cells are 

 strongly thickened. The palisade layer becomes single at the 

 edges, and passes over on the under side of the leaf into the layer 

 of elongated spongy parenchyma (sp'"). The vascular bundles 

 lie in the spongy parenchyma ; the largest, in the mid-rib of the 

 leaflet, extend almost from the inner palisade layer to the lower- 

 most layer of elongated spongy parenchyma. In the vascular 

 bundles themselves, we recognise readily the darker-looking 

 xylem, of vascular tracheides, and the brighter phloem. As the 

 vascular bundle bends outwards from the stem, it follows as a 

 matter of course that the xylem portion is upwards, and the 

 phloem downwards, in the leaf. The radial arrangement of the 

 elements arises from the bilateral, though but temporary, activity 

 of the cambium. Around the bundle is a sheath of parenchyma, 

 the elements of which contain chlorophyll grains, and which join 

 on to the surrounding cells of the spongy parenchyma. The 

 relations of the smaller bundles, of which an example is shown 

 in the figure, are similar. Still smaller vascular bundles (vs), 

 which are reduced to a few vascular tracheides and bast elements, 

 are also met with in the cross-section. These remain to the last 

 surrounded by a sheath of elongated parenchyma. The internal 

 glands (sc) impign upon the epidermis of the upper or under 

 side. They are circular in outline, clothed by a layer of thin- 

 walled, more or less disorganised cells, indicating their lysigenous 

 origin ; to which follows a layer of flat cells with granular con- 

 tents, and fairly thick white walls. The surrounding chlorophyllr 

 containing mesophyll joins on to these cells. The epidermal 

 cells which lie over the gland are flatter than their neighbours. 

 The volatile oil can be readily removed by alcohol, in which it is 

 freely soluble. 



Petiole of Ruta. Surface-sections taken at the base of the 

 general petiole show the epidermis composed of cells elongated 

 in the direction of length of the petiole, and interrupted alike 

 on the upper and under surface by stomata. Oil glands are 

 also present. Under the epidermis lies a layer of elongated, 

 collenchymatous cells, and then follows the chlorophyll-con- 

 taining tissue. In cross-section the epidermal cells are seen 

 to be more strongly thickened on their outer side ; then follows 

 the single layer of thickened coljenchyma, absent only under the 



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