61 THE TRANSVEKSE SECTIONS OF PETIOLES OF EUCALYPTS 



III. — Classification of Tissues. 



Before describing the sections, it will be well to settle the system of classification 

 to be adopted for the tissues. 



Various methods have been proposed, and more or less generally used. Of these 

 we will briefly glance at four of the most important, and indicate the one most 

 convenient for our present purpose. 



One system which has been very widely used is the topographical of Sachs. In 

 this the tissues are mapped out according to their relative positions — the dermal 

 tissue, the ground oy fundamental tissue inside, and the fibro-vascular bundles or 

 fascicular tissue studding the ground tissue. This threefold division is certainly very 

 simple and easily applied, but it is not scientific, for geographical position does not 

 always reveal the essential character of a tissue. 



A second sj'stem is the developmental of Hanstein, and, at first sight, it seems to 

 be thoroughly scientific, being based upon development. In this classification certain 

 meristem layers form certain tissues, and these get special names. There is a three- 

 fold arrangement of the primary layers, as in animals, viz., dermatogen, forming the 

 epidermis ; peribleni, forming the cortex ; and a central cylinder of plerome, forming 

 vascular bundles and pith. But the objection to this system is that it does not 

 always hold good, for, as instances, in perforations of the leaf in many aroids, and in 

 splitting of the leaf into segments in palms, the original epidermis is replaced by 

 another, derived from the underlying tissue. 



The morphological system of De Bary is purely descriptive, and, therefore, the 

 safest to adopt in the present state of our knowledge. We know so little of function 

 as yet that we can do little more than place together parts agreeing in structure, 

 although they may have little else in common, or no physiological relationship. 

 Parenchyma is thus one kind of tissue, sclerenchyma another, and so on, and this is 

 the classification most convenient for present use. 



But convenience is only a make-shift, and sooner or later ftmction and structure 

 together will determine the true position of any system of tissues. This brings us to 

 the last system to be mentioned, viz., the functional of Schwendener and Haberlandt, 

 in which a certain structure is associated with a certain function or functions. The 

 function is here brought to the front, and such terms as bast, cambium, &c., are used 

 to denote tissues performing special functions, and not as is usually the case, tissues 

 having special positions. Certain tissues may have a certain position by virtue of 

 their function and consequent structure, but that is an accidental, and not an essential 

 circumstance. Relative position and structure are not considered apart, but as 

 dependent on function. As already stated, function is as yet so little understood that 



