3i 
Anatomy of the Opkioglossaceae. 
apical pole directed inwards, i. e. away from the archegonial neck. Following 
on this come divisions at right angles to the long axis of the embryo, 
resulting in a stage in which the embryo consists of a row or filament of 
three cells. Further divisions may take place in all three cells, but the 
large cell next the archegonial neck often remains undivided ; it forms the 
upper tier of the suspensor. The next cell undergoes division and usually 
forms the short and wide second tier of the suspensor ; it may, however, 
take the form of an elongated row of cells. The embryo proper is derived 
from the third or terminal cell. The divisions in this result in the dis- 
tinction of a hypobasal half from which the foot is mainly developed, and 
of an epibasal half in which the apex of the shoot is initiated close to the 
central point. By further growth of the epibasal half all the primary 
members of the young plant are laid down, the first leaf developing close to 
the apex of the stem, while the primary root probably arises endogenously 
and originally points backwards towards the hypobasal half and the 
suspensor. The vascular system of the plant is initiated in the axial line 
between the apex of the shoot and the primary root. 
Up to this stage the growth of the embryo would appear to be deter- 
mined by its position in the prothallus with relation to the archegonial 
neck, and the axis of the embryo is approximately straight. The further 
growth to the condition in the mature embryo involves a marked curvature. 
This is not a simple bending, for the whole embryo is growing at once. It 
results in the enlarging shoot becoming vertical. The facts do not allow 
of the full mechanism of this being understood, but two factors must be 
recognized, both of which probably come into play. On the one hand, 
there is the great and unequal enlargement of the hypobasal tier giving rise 
to the foot, which bulges towards and into the prothallus from which 
nutriment has to be obtained. On the other hand, and possibly more 
important, is the change in irritability of the growing shoot in relation to 
gravity ; the shoot ceases to grow in the original line of the primary 
filament of the embryo and grows vertically. Both these factors probably 
contribute to the changed configuration of the mature embryo. The erect 
position of the shoot is not attained passively by the enlargement of the 
foot, but by active growth. On the other hand, the apparently lateral 
position of the first root is probably the result of the great increase of the 
hypobasal region on one side in forming the foot . 1 
1 The distinction between two stages in the development of the embryo, the first in which the 
form and structure is dependent on internal factors and on the correlations between the embryo and the 
surrounding tissues, and the second in which the direction of growth of the organs is dependent on 
their reactions to such external stimuli as gravity, is possibly an important one. It may be pointed 
out that it is seen in the Spermaphytes in (i) the growth of the embryo in the embryo-sac of the 
developing seed, and (2) the growth of the embryo on the start of germination. In this case the 
two stages appear separated by the period of rest. In such a case as the embryo of Helminthostachys 
the two stages are recognizable, but the development goes on without a break. The interesting 
