THK CANADIAN H0KTICULTUKI8T. 



183 



THE LEAF AS A STUDY. 



At a meeting of the Massachusetts 

 Horticultural Society, Dr. G. Austin 

 Bowen, of Woodstock, Conn., read a 

 paper on "The Leaf as a Study." 

 After speaking of the apparent insigni- 

 ficance, to general view, of a leaf, he 

 passed to the consideration of it from 

 the point of view of the physiologist, 

 who looks on it as a part of matter 

 having its function to perform, and its 

 relationship to other great creative 

 powers of nature. Before we have the 

 leaf we have the bud, which consists of 

 a brief cone-shaped axis with a tender 

 growing point, bearing a protective 

 covering of imbricated scales and inci- 

 pient leaves. Within this bud the 

 botanist will point out the rudimentary 

 leaf and bud envelopes, and classifies 

 with the greatest minuteness the vari- 

 ous characters therein presented. A 

 day could be pi-ofitably spent with him 

 in consideiing the vernation of the bud. 

 With the coming of spring the bud 

 swells and throws off its now useless 

 scales, and we have the miniatvire leaf, 

 tender in its fibre, delicate in its tracery 

 of outline, and beautiful beyond the 

 painter's art in its softened coloring. 

 A few days of warm sunshine, and 

 every shrub and tree is loaded with 

 verdure ; hundreds of thousands of tons 

 of foliage liave appeared — from whence? 

 The leaf is fullgrown, and from now to 

 fall cai'ries on its peculiar functions, 

 which present questions of intense in- 

 terest to the stiulent of JSTatui-e. 



Anatomically considered the leaf is 

 an expanded portion of the substance 

 of the bark, extended into a broad, 

 thin plate, by means of a woody frame- 

 work or skeleton, issuing from the 

 inner part of the stem, and called the 

 lamina, or blade of the leaf, and con- 

 sisting of two parts, the framework 

 and the parenchyma. The framework 

 is made up from the branching vessels 

 of the footstalk, which are woody tubes, 



pervading the parenchyma, and carry- 

 ing nourishment to every part. From 

 the analogy of their functions these 

 vessels are called veins. The paren- 

 chyma consists of two parts, or strata, 

 moi-e or less distinct, and arranged dif- 

 ferently in leaves, whose natural posi- 

 tion is horizontal or vertical. Externally 

 the leaf is covered with a layer of empty 

 united cells, mostly tubular, forming a 

 superficial membrane, called the epider- 

 mis, and is analogous to the cuticle that 

 covers our own bodies ; its ofiice in the 

 leaf is to check evaporation. The por- 

 tion of the parenchyma immediately 

 beneath the epidermis — the upper side, 

 or that which faces the sun — is com- 

 posed of one or two layers of oblong- 

 cells placed perpendicularly to the sur- 

 face, and more compact than the layer 

 of cells beneath them, which constitutes 

 the lower stratum, and which also con- 

 tains, in common with the whole epi- 

 dermis, the stomata or mouths, which 

 are little clefts tlirough the epidermis, 

 and are always placed over and com- 

 municate with the inter-cellular pass- 

 ages. These little openings are guarded 

 by valves, which are supposed to regu- 

 late transpiration. The number of 

 these stomata is astonishing ; a single 

 square inch of surface of the leaf of our 

 common garden rhubai'b contains 5,000, 

 the garden iris 12,000, the j)ink 36,000, 

 and the hydrangea 160,000. Our leaf 

 also possesses glands, which are cellular 

 structures serving to elaborate and con- 

 tain the peculiar secretions of the plant, 

 such as aromatic oils, resins, honey, 

 poisons, etc. 



The beautiful green of the leaf, so 

 restful to the eye, is a waxy substance, 

 termed chloroi)hyl ; which floats in the 

 fluid in the cells as minute granules. 

 The light of the sun seems to be essen- 

 tial to its formation. Although found 

 in vast abundance in the vegetable 

 kingdom, hundreds of tons being evei'v 

 where around us in the growins: season. 



