THE EMBRYO 279 



After the two-celled stage (Fig. 151 A) the basal cell cb divides trans- 

 versely to produce ci and m (Fig. 1515) and the terminal cell divides 

 to produce I and V (Fig. 151C). Following this four-celled stage, 

 m and ci divide to produce the cells d,f, n, and n' (Fig. 151D), each 

 of which may divide again, resulting in a suspensor which is about 

 8 to 12 cells long. At the same time the cell V segments into ph and 

 h (Fig. 151D) after which I and ph divide by longitudinal walls 

 oriented at right angles to each other, and h divides by a transverse 

 wall. Two juxtaposed cells are thus produced from I and ph and 

 two superposed cells (ha and Kb)' from h (Fig. 151E). The embryo 

 proper (excluding the suspensor cells) now consists of four tiers, I, 

 ph, ha, and Kb. In I and ph the next division is vertical and in a 

 plane at right angles to the first division. At the same time ha also 

 divides by a vertical wall, resulting in two tiers of four cells each at 

 I and ph, one tier of two cells at ha, and one of a single cell Kb (Fig. 

 151F). In the cells of the apical tier /, the next division walls are 

 diagonal, followed by periclinal divisions which cut off the dermat- 

 ogen. The cells of the tier ph divide periclinally; the outer cells 

 form the dermatogen, and the inner undergo further longitudinal 

 divisions to form the periblem and plerome initials of the stem 

 (Fig. 151 G). The two cells of the tier ha give rise to a single semi- 

 circular layer of cells which contributes to the periblem, the dermat- 

 ogen, and a part of the root cap (Fig. 151H-J). The cell Kb divides 

 only at a comparatively late stage, at first by a transverse wall and 

 then by a vertical wall in each of the daughter cells. The upper 

 derivatives form a part of the root cap, which is supplemented on all 

 sides by a cell from the tier ha and in some cases by extra cells cut 

 off from the dermatogen of tier ph. A peculiarity of the suspensor 

 is the widening and vertical divisions of two or three of its cells 

 lying just above the embryo proper (Fig. 151//). 



In Sherardia (Soueges, 1925), a member of the Rubiaceae, the 

 four-celled proembryo arises in the same way as in Nicotiana and 

 Lobelia. The first division in the cells / and V (Fig. 152A) may be 

 either transverse or vertical, but m and ci always divide transversely 

 (Fig. 1525). The eight-celled stage may thus comprise six, seven 

 (Fig. 152D), or eight (Fig. 152C) tiers of cells. Normally the de- 

 rivatives of I form the cotyledonary region and those of V form the 

 hypocotyl; m and ci give rise to a massive suspensor, which is fila- 

 mentous towards the apical end but is composed of a number of 

 large vesicular cells at the basal end (Fig. 152E-G). . Presumably 



