SECT. I 



MORPHOLOGY 



133 



converge (Fig. 142). In this way several palisade cells come into 

 direct contact with a single expanded cell (s) 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. The 

 sheaths serve as a conducting tissue towards the stem. 



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

 the leaf-stalk, where one is present. The dorsiventral structure 

 becomes less marked in the petiole. The cells are mostly elongated 

 in the length of the petiole, a modification which facilitates the con- 

 duction of food material. They are often thickened and so arranged 



Fio. 142. — Transverse section of a leaf of Fagus sylmtica. ep, Epidermis of upper surface ; ep", 

 epidermis of under surface ; ep"', elongated epidermal cell above a vascular bundle ; pi, palisade 

 parenchyma ; s, collecting cells ; sp, spongy parenchyma ; fc, idioblasts with crystals, in k' 

 with crystal aggregate; st, stoma, (x 300.) 



as to meet the altered requirements for mechanical rigidity. The 

 vascular bundles pass from the leaf-stalk into the stem and there 

 either arrange themselves among the bundles of the central cylinder 

 or at once fuse with some of them. In the leaf-stalks of Angiosperms 

 the bundles usually appear arranged in a curve open above, but may 

 form other figures. In the petioles of Ferns, the partial cylinders are 

 accompanied, as in the stem, by sclerenchymatous fibres forming 

 strands or plates. It is the peculiar arrangement of those brown- 

 walled sclerenchymatous masses which forms the double eagle apparent 

 on cross-sections of the petiole of Fferis aquilina, from which the plant 

 derives its specific name. 



In certain families of the Dicotyledons, particularly in the Crassulaceae, the 

 mesophyll of the leaf-lamina forms peculiar masses of tissue called the epithema 

 between the swollen terminations of the bundles and the epidermis. The cells of 

 the epithema are small and, for the most part, devoid of chlorophyll ; they are full 

 of water, and joined closely together, leaving only very small interspaces, which are 



