OF SOUTH AUSTRALIA. 
121 
cell and the cells of the sterile filaments into the one large, irregularly- 
shaped placental or central cell. The carpogonial filament withers and 
plays no part in the nutrition of the developing carposporophyte, which 
when mature is not freely exposed, but is closed over by a flask-shaped or 
urn-shaped envelope, the pericarp, with a conspicuous ostiole at its distal 
end. Actually the pericarp begins to develop even before fertilization 
from pericentral trichoblast cells adjacent to the supporting cell, and the 
whole fruiting structure of pericarp, supporting cell, auxiliary cell 
and gonimoblast filaments or carpogonial filaments and carpospores is called 
a cystocarp. 
Carpospores are profusely liberated from the cystocarp via the ostiole 
of th'e pericarp and are carried passively by water currents. They soon 
fall to the sea floor and in a few hours begin to divide mitotically, the 
nucleus retaining the diploid number of 40 chromosomes, and eventually 
develop into mature thalli which bear neither spermatangia nor carpo- 
gonia. Cytologically this sporophyte thallus differs from the gametophyte 
thallus which bears sex organs (although they are not morphologically dis- 
tinguishable) in that tetraspores are produced in unilocular tetrasporangia. 
The tetrasporangium is formed from a special pericentral cell cut off 
laterally from axial cells of a ramulus, and rows of tetrasporangia are 
produced on successive tiers. The fertile pericentral cell first cuts off a 
daughter cell on its outer face, and this in turn cuts off a pair of cover 
cells at the upper face, and in some species a smaller peripheral cell in 
addition. The fertile pericentral cell divides once transversely, and of the 
two daughter cells formed, the lower is the stalk cell and the upper 
becomes the tetrasporangium, and this increases in size to become quite 
large. The chromosome number of the single nucleus is diploid, and 
by reduction division four tetrahedrally disposed haploid tetraspores are 
formed. On rupture of the sporangial wall the mature tetraspore escapes. 
Its germination is very similar to that of a carpospore except that the 
nucleus contains only 20 chromosomes, the haploid number, and so gives rise 
to a gametophyte thallus which bears sex organs. Diagrammatically this 
life-cycle can be represented as in Fig. 2. 
(c) Modifications in Other Life-cycles. — The typical reproductive cells 
of the Euflorideae are the non-motile gametes which fuse to form a fer- 
tilized egg (zygote) which never separates from the mother-plant but which 
remains in intimate contact with it and is nourished by it to produce the 
so-called gonimoblasts which in turn bear carpospores. On germination 
these give rise to new individuals. In Nemalion these are the normal 
gametophytic thalli which bear sex organs and so complete the life-cycle, 
but in Polysiphonia these individuals are sporophytes and undergo a reduc- 
tion division to produce tetraspores from which the gametophytic thalli 
arise. Thus there are two types of alternation of generations and Svedelius 
calls the former type as in Nemalion where no tetraspores are formed, the 
haplobiontic type, and the latter type as in Polysiphonia , where there is 
a regular alternation between individuals bearing gonimoblasts and 
