EMBRYOGENESIS IN THE ALGAE 55 



rapidly, separate from the hair and form a rapidly-widening funnel- 

 shaped depression round it. Cells adjacent to the initial cell also grow 

 out as hairs, so that the embryo is characterised by an apical tuft of 

 hairs. The first hair soon becomes disorganised and shrivelled but its 

 initial (i.e. basal) cell persists, and it is this cell which thereafter con- 

 stitutes the apical cell proper of the embryo. As Fritsch (1945) points 

 out, the origin of the apical cell below a trichothallic meristem recalls 

 the condition met with in certain Ectocarpales. It is not at present 

 known whether the apical cell originates in a similar manner in other 

 Fucaceae. In Pelvetia and AscophyUiim, the young embryos do not 

 show the characteristic distal tuft of hairs seen in Fuciis. 



All young embryos in the Fucales thus far investigated have a 

 'three-sided' apical cell lying in a distal depression. Actually the cell 

 is like the basal portion of a truncated three-sided pyramid, the base 

 being directed inwards: in longitudinal section it appears to be 

 biconvex. This cell cuts off lateral segments which, on dividing by 

 periclinal walls, give rise to the peripheral and inner tissues of the 

 thallus. 



At an early stage in the embryogeny of Fucus, the distal region of 

 the thallus begins to flatten and, as growth proceeds, the strap-like 

 soma is seen to have a midrib, while the apex occupies a slit-like 

 depression extended in the plane of flattening. At this stage, probably 

 in relation to the inception of bilateral symmetry, the three-sided apical 

 cell is transformed into a four-sided one, i.e. it is now like the basal 

 portion of a four-sided pyramid. It is thought that this definitive apical 

 cell arises from the three-sided one by longitudinal division. In Fucus, 

 the four-sided apical cell gives rise successively to basal segments, 

 which form the central medulla, to segments from the narrower faces, 

 and then to a segment from one, sometimes both, of the broader lateral 

 faces. This last segment is often as big as the apical cell and, in fact, 

 it affords a histological basis for the bifurcation of the thallus, i.e. the 

 lateral segment becomes an apical cell. As each of the two apical cells 

 cuts off a considerable number of segments, they become separated and 

 give rise to the two limbs of the dichotomy. The lateral segments 

 collectively give rise to the inner and outer peripheral tissues. 



Lastly, in considering how the characteristic organisation of the 

 plant becomes manifest during the ontogeny, it should be noted that 

 in the Fucales it is not enough to study the apical cell and its characteris- 

 tic divisions : in these algae, as in the ferns, a well developed superficial 

 apical meristem, with distinctive component cells, becomes organised 

 during the ontogenetic development, the formation of the lateral 

 members, and the orderly development of the plant as a whole, being 

 referable to this meristem. There is, in fact, a quite remarkable 



