6 INTRODUCTION. 



the ordinary use of the words : a sharper definition of them is here neither necessary 

 nor possible. When a layer (simple or compound) surrounds a tissue, which differs 

 from it, it is termed relatively to the latter a sheath. Tissues of whatever sort or rank 

 may be mutually continuous for long distances, or throughout the whole plant. When 

 this is the case, they are said collectively to form a System. A system of any rank 

 may be composed of systems of lower rank, or may join with others to form a 

 system of higher rank. For instance, the Vascular system is composed in most 

 plants of that of sieve-tubes, and that of vessels ; each of the latter is a system of 

 itself ; the two combine as a rule to form the above-named system of higher rank ; 

 while often the system of Sclerenchyma-fibres joins them as a third constituent of the 

 joint system. 



According to the point of view from which one starts, one may therefore 

 distinguish systems in the most various sense, as will appear in the later chapters ; for 

 instance, in the Dicotyledonous stem we may with equal right distinguish a Vascular 

 and an Epidermal system, or a Woody and Cortical system, whereas the latter includes, 

 besides the Epidermal tissue, a part of the Vascular system, and other tissues besides. 



Sachs (Textbook, Eng. ed. 1882, p. 79, etc.), in the exposition of the anatomy of the 

 higher plants, starts from the definition of three systems of tissue, which he terms Dermal, 

 Fascicular, and Fundamental tissue. Under the first term he includes those tissue-forms, 

 which limit externally such plants as have their cells aggregated in three dimensions 

 of space, as a matter of fact, Epidermis and Periderm (cf. § 2, 23 and chap. xv). 

 His Fascicular tissue corresponds in the main to the previously mentioned Vascular 

 system (chap. viii). The name Fundamental tissue includes what remains after the 

 separation of the other two. However much this distinction may be fitted to guide 

 beginners, still, in my opinion, it does not answer its purpose, which is to serve as a basis 

 for a uniform exposition of the various differentiation of plant-tissues. For the names 

 Dermal and Fascicular tissue indicate in Vascular plants systems of tissue, which are 

 positively characterised by definite tissue-forms : but the name Fundamental tissue im- 

 plies the remainder, and this may just as much consist of different positively characterised 

 tissue-forms, and tissue-systems, which are equivalent to the Dermal and Fascicular 

 systems. But if it is necessary, in the description of the Dermal and Fascicular systems, 

 to make use of a short general term for the tissues over and above these, the terms 

 Fundamental tissue, or Fundamental mass, or Intercalary mass, are very suitable ; just 

 as in Nageli's treatise on the vascular bundles, or fibro-vascular masses, was his distinc- 

 tion of these from the rest ('Proten'), or as was Schwendener's general term for the 

 parts of the vascular bundle, which, in his exposition of the mechanical adaptation, did 

 not bear upon his point. And indeed in describing a form or system of whatever rank, 

 some such method must always be used. I think, however, that the distinction of the 

 forms of tissue must first serve as a foundation for the uniform exposition of the subject 

 which now engages us, and for the choice of terms; then only should follow the investi- 

 gation, how far these forms of tissue take part in the formation of combinations, and 

 systems of higher rank. 



Although on the one hand the course of exposition above brought forward has 

 a definite justification in all cases, and, on grounds which need not be repeated, will 

 be in the present case pursued, and although further the distinction of single forms of 

 tissue must be carried out under all circumstances with regard to the structure of the 

 elements only, still the question arises, on the other hand, if the distinction of 

 definite systems could not be drawn more naturally on other than the purely histo- 

 logical grounds above stated ; that is, on such as are derived from the morphology 



