INTRODUCTION. 17 



Filices and Marsileaceje — we may distinguish in the first stages of the further 

 development of a segment, three sorts of division, diffe:"''"^' iii their direction and 

 resuhs. They are : — 



(i) Divi'siofis into flats (Etagentheilungen), that is, splitting of the segment into 

 similar stories one above another by division walls at least approximately parallel to 

 the principal walls. 



(2) Radial halvitig, division of a segment into halves lying side by side, but 

 never quite alike, by an approximately (but not exactly) radial wall : in the case 

 of segments arranged in two series, that is, which correspond in the circular transverse 

 section to semicircles, this radial halving divides the section into (unequal) quadrants; 

 in case of segments in three series, into sextants; the walls in question are named 

 accordingly. In the first case the division into quadrants is followed either by 

 a second halving by octant-walls (stem of Salvinia, AzoUa) or only each larger 

 quadrant is again halved, so that each segment is divided by two radial walls into 

 three cells (stem of Selaginella Martensii). 



(3) Division into strata (Schichtentheilung), that is, division by tangential walls 

 into concentric strata parallel to the surface. 



These three modes of division, which appear as the first successive stages of 

 division, are followed by further divisions in the three principal direcdons in each 

 story, and in each stratum. These divisions alternating variously according to the 

 species, finally result in the definitive composition of the segment. 



Of the above three first stages of division, that marked (3) is seldom the first. 

 Using these figures as above to express them shortly, they usually appear in the 

 succession i, 2, 3 (apex of the stem of Equisetum, Salvinia), or 2, 3, i (root of 

 Ferns), 2, i, 2, 3 (apex of stem of AzoUa) ; only in the root of Azolla the suc- 

 cession 3, 2, &c. Avas found by Strasburger. Relatively to the future layers of 

 meristem, i.e. to the later developed tissue-layers, the first products of division of 

 the segments are thus, with the exception of the last mentioned case, still common 

 initial cells. 



From the division into strata, marked (3) above, arise layers of meristem, which 

 correspond in their arrangement to the three principal layers of the root of the 

 Angiosperms, i. e. plerome, periblem, and dermatogen. In many cases, though not 

 in all (e. g. in the roots of Ferns and Equisetum), these undergo a similar develop- 

 ment to that of the similar layers of corresponding members of the Angiosperms. 

 They are usually sharply defined, since the walls separating them (like other longitu- 

 dinal walls) in the successive segments fit pretty accurately one on another. As is 

 evident from what has been said, we have to deal, in the phenomenon in question, 

 with more than one, at least two, successive divisions. 



As is evident from the foregoing account, many diflferences peculiar to special 

 cases obtain in the very first stages of division. This is the case to a still greater 

 extent in the later stages, which bring about in the segments their definitive compo- 

 sition. To enter with uniform minuteness into the peculiarities of individual cases 

 would lead us much too far. After referring to the special literature, and particu- 

 larly to Strasburger's description of the many peculiarities of Azolla, we need cite 

 here only a few examples, keeping an eye at the same time upon many relations of 

 form, which have not been touched upon in what has gone before. 



c 



