INTRODUCTION. 7 



of differentiation (i.e. the history of development). Investigation has taught that the 

 original meristem, from its first (embryonic) beginning onwards, maintains a well de- 

 fined differentiation into different — but always meristematic — layers, or segments; and 

 that, in the plants on which we are engaged, in many cases definite sharply marked 

 masses of tissue (I may mention provisionally the axile vascular bundles of many 

 roots) are derived from certain of these layers, others from other layers, just as the 

 different tissue-systems of the animal body are derived from the different germinal 

 layers. The question may therefore arise whether, over and above the distinction of 

 forms of tissue according to definite structure, the exposition of the tissue-systems, 

 and of the construction of the members of the highest rank from them, must not refer 

 to that differentiation of meristem, and take it as the foundation. The determination 

 on this point will depend upon the answer of the further question, whether the origin 

 of each, or of single tissues or tissue-systems, from one and the same portion of the 

 primary meristem, can or cannot be generally proved. For if the latter is the case, if 

 parts of the same tissue-system originate from unlike parts of the meristem, the 

 course of exposition which we are considering must be put aside as impracticable. 



In order to gain clearness on this question, a survey of the original modes of 

 differentiation of meristems is necessary; and though this lies beyond the strict limits 

 of the subject of this book, still it may with the more reason be here inserted, since 

 in the following chapters reference must constantly be made to that differentiation '. 



I. As Hanstein ^ has shown, the young embryo of the Angiospermous Phanerogams 

 separates, while still consisting of few cells, all-cf which are meristematic, into three 

 layers, or groups of cells, which differ in their arrangement and direction of division ; 

 these were termed by their discoverer, Dermatogen, Periblem, and Plerome', At 

 the end of the root there is besides these a fourth, the origin of which in the embryo 

 remains to be further investigated. This was called by Janczewski * the Calyptrogen 

 layer : we shall return to this later. The dermatogen layer is separated by a single 

 tangential division of the few cells, which forni the original rudiment of the 

 embryo, as a simple peripheral layer of cells. It remains a simple layer of cells, since 

 all the subsequent meristematic divisions which occur in it take place only by walls at 

 right angles to the surface. It is only at the future apex of the root that other pheno- 

 mena appear, which need not be more fully discussed at present. Further divisions 

 of the cells enclosed by the dermatogen separate an axile longitudinal cylinder, the 

 plerome, from the periblem, which is a zone of tissue lying between the plerome and 

 dermatogen. The pletome consists of cells in which longitudinal divisions pre- 

 ponderate, and which have a corresponding arrangement; the periblem of cells 

 distinguished from the former by more frequent and irregular transverse divisions. 



' [See further Sachs, Ueber die Anordnung d. Zellen in d. jiingsten Pflanzentheilen, Arb. d. Bot. 

 Inst, in Wiirzburg, Bd. II; also, Ueber Zellanordnung u.Wachsthum, ibid. — Haberlandt, Scheitelzell- 

 wachsthum b. d. Phanerogamen, Botan. Zeitg. 1882, p. 343. — Nageli, Scheitelwachsthum d. Phanero- 

 gamen, Naturforscher Vers, zu Miinchen, 1877.] 



" Hanstein, Botan. Abhandl. I. On the details of the origination of the embryo, and its great 

 differences in those Di- and Monocotyledons which have been investigated, the reader must be 

 referred to this treatise and to Sachs' Text-book, English edition, 1882, pp. 585-593. Further, 

 Fleischer in Flora, 1874, p. 369, &C.4 Hegelmaier, Bot. Zeitg. 1874, p. 631, &c. 



' Hanstein, Die Scheitelzellgruppe im Vegetationspunkt der Phanerogamen, Bonn, 1868. 



* Ann. Sci. Nat. g serie, torn. XX. 



