Lecture XXVII. 245 



and usually, the number of elements being more limited, there is 

 greater regularity in their arrangement. As a tract or bundle is 

 traced downward it is found that the wood and bast are further 

 reduced while the sheath increases in thickness. The conducting 

 tracts of the flowering stem pass down and unite with those of the 

 stem of the bulb so that the whole conducting system is continuous 

 throughout the plant. 



The leaves of the wild Hyacinth are constructed very much in 

 the same way as those of the Buttercup. They consist of a middle 

 layer (mesophyll) of green cells put together loosely and covered 

 over, above and below, by a colourless epidermis. The epidermis 

 also is coated with a cuticle, but it has no hairs and presents, conse- 

 quently, a smooth and burnished appearance. The mesophyll is 

 somewhat more porous towards the under surface, but the differentia- 

 tion into palisade and spongy parenchyma is scarcely apparent. In 

 the disposition of the conducting tracts in the leaves of the two plants 

 there is a notable difference. In the Buttercup the conducting 

 tracts branch in the mesophyll and form very fine branches which 

 anastomose, or join together, irregularly and form a network in the 

 leaf. In the Hyacinth the chief conducting strands run parallel to 

 the mid-rib and here and there send off oblique branches uniting 

 with a neighbour. This difference is characteristic of the two 

 groups to which the plants belong. As the large conducting tracts 

 lie in the veins of the leaves, we find that parallel venation is 

 characteristic of the leaves of the Monocotyledons while reticulate 

 (netted) venation is characteristic of the leaves of Dicotyledons. 

 The arrangement of the stomata on the leaves constitutes also a 

 method by which the epidermis of a Dicotyledon may be generally 

 distinguished from a Monocotyledon. In the former the slits of 

 the stomata are oriented fortuitously, in the latter they are parallel 

 to the mid-rib. There are stomata on both the upper and lower 

 epidermis of the Hyacinth. 



The structure of the flowers must next engage our attention. 



The flower-stalk is continued upward as the axis, to which the 

 floral leaves are attached. In the Hyacinth the outermost floral 

 leaves form a whorl of six. They are blue in colour l and each is 

 ovate in shape. The distal tapering end of each is recurved so 

 that the whorl forms a structure like a vase surrounding the inner 

 parts of the flower, and performs not only the protective functions 

 of the calyx but also the attractive functions of the corolla. Such 



1 The pigment in this case is not held in chromatophores, but is dissolved 

 in the vacuoles of the cells. 



