INTRODUCTION. 



^9 



simplest cases further divided by a tangential wall [e) into two cells, of which the outer is 

 the initial of the dermatogen, the other of the periblem. The dermatogen remains in the 

 simplest cases a simple layer, since its cells suffer only radial divisions alternately hori- 

 zontal and vertical, but it may also be once more divided tangentialiy. The two other 

 layers are further divided and developed by successive vertical walls in definite order, 

 which need not now be followed ; later these are accompanied by transverse divisions 

 into flats (Etagentheilung). In roots which increase much in thickness each outer cell, 

 after the appearance of the wall c, may further divide by a radial vertical wall intc two, 

 in which the division by the wall e then takes place. 



riG. 9. — Apex of the stem of Equisetum. A (530) longitudinal section through a strong shoot of E. telmateja, • 



B view of the apex of such a shoot from above (Sachs); C,D, E, from H. arvense, after Cramer ; C a diagrammatic 

 gfround plan of the apical cell and of the youngest segment, Z> optical longitudinal section of the apex of a stem, 

 Jz transverse section through I \n D, S in all cases the apical cell. The Roman figures I, II, . . . (in B read IV for ' 



VI) indicate in a 1 cases the segments ; the Arabic figures i. 2, 3 . . . the successive division walls within a single 

 segment; the letters rt, *,.■.. in C and Z> the successive principal walls. In ^, jir,_v indicate the hii^hest. youngest 

 rudiment of an annular swelling which will develope into a leaf-sheath ; db the same older, bs apical cells of a stiil 

 older leaf-rudiment, g rows of cells from which the vascular bundles are derived, z'the lowest layers of cells of the 

 segments, r the rudiment of the cortex of the internodes ; the broad central band between i^ and g the plerome. 

 Fr m Sachs' Textbook. 



In the apex of the stem of species of Equisetum (Fig. 9) each segment-cell is, according 

 to Cramer, Reess, and Sachs, divided first of all parallel to the principal walls into two nearly 

 similar stories (B,C,Z)), then follows in each of these the division into alternately dissimilar 

 sextants {F) as in the root ; abnormally in many cases (comp. Reess, Pringsh. Jahrb. VI, PI. 

 X. 8) two sextant walls curved in different directions appear in one segment. The next 

 division in the sextants fe-either a tangential perpendicular one into an outer and an inner 

 cell, corresponding to the scheme for the root, or one not exactly radial and perpen- 

 dicular, which is then followed by the tangential division (/?). Both cases happen side 



c 2 



