ANATOMY OF THP:: LEAF. 



BY SPENCER COBBOLD, ESQ., M. D. 



A LEAF is nothing more or less than a lateral expansion of the outer layer 

 or bark of any stem or bough. An ordinary leaf to the naked eye appears 

 to be merely a flat plate of cellular substance, which, traversed by numerous 

 veins or nerves, radiating from a common vein, or midrib, as it is called, 

 forms that beautiful reticulated structure so familiar to us all. Common as 

 the appearance is, we cannot fail to recognise in it evidence of design. We 

 shall not stop to comment upon the obvious necessity for such an arrangement; 

 this would lead us into a department of the subject to which the term 

 Morphology is more strictly applicable j moreover, it is a branch of inquiry 

 which has only very recently been broken up. 



Let us look more closely at the ultimate elements of which it is composedj 

 this we are enabled to do by aid of the microscope, an instrument which of 

 late years has been brought to great perfection, and at the present time forms 

 one of our most valuable accompaniments in the investigation of natural 

 phenomena. A leaf, then, such as we have already defined, consists, like all 

 other parts of a plant in the fully-developed state, of only two kinds of structure, 

 which present, however, various modifications under different circumstances. 

 These two structures are termed cells and vessels. The cells are merely little 

 bags or vesicles, the wall or covering of which is a transparent homogeneous 

 membrane, of extreme tenuity, and is chemically closely allied to starch. Their 

 size varies considerably; in some plants measuring the twentieth or even the 

 tenth part of an inch in diameter, while in others they are less than the 

 two-thousandth part of the same metre. Their shape is equally variable, which 

 is owing to the manner in which they are compressed together, in this way 

 forming either cubical, pentagonal, octagonal, or dodecahedral cellular tissue. 



Let us now see what is contained within these vesicles or cells. All of 

 them contain a more or less palpable fluid, which in some is watery or still more 

 dense; in others there exists only a vapour; but besides this, ^ 



all those cells, filling up the interstices hctiveen the veins, con- c o o O* »* o 

 tain a number of little round bodies, some of almost incon- ^ ^o^^^iPo 

 ceivable minuteness, termed molecules and granules. For the O ° O e"© 

 sake of distinction we call those granules which present a 

 clear central space, surrounded by a dark zone; while those which are termed 

 molecules, are mere spots or points of variable magnitude. When the larger 

 bodies present a green appearance, they are termed chlorophille 

 granules. These bodies sometimes exhibit under the microscope, •/•'•**o*.,*!*. 

 a series of extraordinary movements, a constant and rapid :.■'■'.•••.•/•,*.' 

 shifting and interchange of position, owing, doubtless, to cer- ;,•... ; -•*.•/. 

 tain attractive and repelling properties, which seem essential 

 to the constitution of all matter. 



As I mentioned just now the existence of Chlorophil in the interior of the 



VOL. II. E 



