116 BOTANY 



palisade cells come into direct contact with a single expanded cell of the 

 spongy parenchyma, which thus functions, apparently, as a collecting 

 cell for a group of palisade cells. The products of assimilation are 

 passed on from the collecting cell through the spongy parenchyma, 

 to be finally carried to the mesophyll sheath surrounding the vascular 

 bundles. 



In the cross-section (Fig. 130) of a leaf of the Beech {Fagus silvatica) only a 

 small vascular bundle is shown. The large bundles are so surrounded by elongated 

 collenchymatous cells that they appear as projecting ribs on the under side of 

 the leaf. In other leaves the vascular bundles, especially on the phloem side, are 

 accompanied by sclerenchymatous fibres. Other strands of sclerenchyma which 

 are independent of the vascular bundles are often met with in the hypoderma. 

 Single (k) and aggregate crystals (&') are also present in the mesophyll of leaves. 

 Often, as in the case of the Beech, cells containing single crystals accompany 

 the bundles throughout their whole course. In addition to crystal cells, all the 

 other forms of secretory cells and glands may exist in leaves. 



At the base of the lamina the tissues close together and pass into 

 the leaf petiole. The dorsiventral structure of the leaf becomes less 

 evident in the petiole ; the cells become more elongated, either for 

 the better performance of their conductive functions, or to enhance 

 the mechanical rigidity of the tissue. In Angiosperms the partial 

 cylinders of the leaf, usually an odd number, and each containing a 

 single vascular bundle, arrange themselves in regular order as they 

 pass through the petiole, and frequently form a bow-shaped figure, 

 opening upwards. On entering the stem the vascular bundles of the 

 leaf join the vascular bundles of the central cylinder ; the fundamental 

 tissue of the leaf-cylinders becomes, similarly, united with the corre- 

 sponding tissue of the central cylinders. In the petioles of Ferns, the 

 partial cylinders are accompanied, as in the stem, by sclerenchymatous 

 fibres. It is the peculiar arrangement of these brown-walled scleren- 

 chymatous plates which forms the double eagle apparent on cross- 

 sections of the petiole of Pteris aquilina, and from which it derives its 

 specific name. The partial cylinders of the leaves of Pteridophytes 

 also join the partial cylinders of the stem, and their corresponding 

 elements become united. 



The Course of Vascular Bundles. — The bundles maintain a definite 

 course and arrangement within the body of a plant. It is some- 

 times possible, by maceration, to obtain preparations in which the 

 course taken by the bundles may be followed. Similarly, by allow- 

 ing a leaf, stem, or flower to lie in water until it has become softened 

 and disintegrated, a skeleton formed by the more imperishable vascular 

 system may be obtained. 



Vascular bundles which pass from a leaf into a stem, and continue 

 for a distance in a distinct course, are called leaf-traces. The leaf- 

 traces may be composed of one or more vascular bundles, and are 



