INTRODUCTION. 5 



reference to the sieve-tubes ; it remains doubtful whether the contents of these are 

 protoplasm or not. The cell-walls of the elements in question are wholly or mostly 

 retained. 



According to the differences already mentioned, which will be further followed 

 out in the subsequent special observations, the tissues divide themselves into those 

 ivhich consist permanently of cells (cellulse), and those whose elements are descend- 

 ants, derivatives, or products of change or metainorphosis of cells. According to theif 

 form and other properties these are termed tubes (Tubi, Tubuli), sacs (Utriculi), 

 fibres (Fibras), and are distinguished from cells. 



Most tissue-elements, of whatever sort, are formed directly and quickly by the 

 metamorphosis of meristematic cells. Exceptions from this rule only occur in certain 

 cases when cells, after they have acted as such for a long time — even for years — may 

 secondarily pass over to another tissue-form. This takes place in the secondary 

 development of Sclerenchyma, which will be described in chapters II. and XV. 



From this secondary metamorphosis of tissue we must distinguish death, and the 

 changes eventually connected with it, which appear in certain other cases in the 

 tissues, such as the dying off of old hairs, cork-cells, cells of the pith of many plants, 

 of the elements of bark, and of the old wood of Dicotyledons, &c. ; appearances 

 which can usually be distinguished with ease from metamorphosis of tissue by the 

 commencement of rotting, weathering, &c. 



In accordance with the preceding considerations, the distinction of the forms of 

 tissue which act as vegetative organs, and the classification of the study of them, 

 must in the first place be founded upon their structure, that is, on the structure of 

 the single tissue-elements, and the connection of these with one another — whether 

 they be connected with like or with unHke elements. It is obvious that, in organized 

 bodies, certain peculiarities and varieties of structure are connected with certain 

 phenomena of development. And it is not less obvious that varieties of structure 

 are also as a rule correlated with certain varieties of form of single tissue-elements. 

 But experience teaches that between form and structure a conslajit relation does not 

 exist, or at least not universally ; and that, contrary to the older classifications, which 

 regarded in the first place the form of the elements, this is of only secondary im- 

 portance in the distinction of the tissues. 



According to the principles now laid down, the following main forms of 

 vegetative tissue may first be distinguished : — 



I. Cellular tissue, i. e. that which consists of permanent typical cells ; with the 

 main subdivisions, epidermis, cork, parenchyma. II. Sclerenchyma. III. Secretory 

 structures. IV. Vessels. V. Sieve-tubes. VI. Milk-tubes. Separate notice of the 

 Intercellular spaces may, with advantage, though perhaps not necessarily, be appended 

 to the study of the tissues. 



The study merely of the divisions and subdivisions of tissue-form is at all 

 points closely connected with the arrangement of these into combinations of 

 different rank — so as to form vegetative organs of successively higher rank — so far 

 so, that the two modes of study can never be completely separated the one from 

 the other. 



According to their form the combinations of any rank may be distinguished as 

 Layers, Bundles, Masses {Groups, Nests) — terms, the meaning of which is obvious from 



