DEVELOPMENT, 73 
green gems which add so great a charm to the 
branching forms which they clothe. As the twig 
is a continuation of the branch in the same way 
that the latter is a continuation of the stem or 
trunk, so the leaf is a continuation, but a con- 
tinuation with expansion, of the tissues of the 
twig. The framework of the leaf—its veins or 
nerves, which spread out like a network—consists 
of fibro-vascular bundles, as they are termed, that 
is to say, of tissues formed by a commingling of 
of wood, bast, and vascular tissues. The softer 
supervening portion of the leaf, that which is 
spread out like a garment over the fibro-vascular 
framework, is made up of cellular tissue. The 
forms of these beautiful expansions of the tissues 
of plant stems are in number infinite as the 
infinite variations in the loveliness of Nature. 
Their purpose is to afford the largest possible 
surface for the beneficial and beautiful operations 
of air and sunshine. 
Air and sunshine, indeed, are so largely con- 
cerned in the development of Trees as to 
contribute, by the elements of the one and the 
influence of the other, no less than one-half of 
