IN ANGIOSPERMS 43 
system of root and shoot. A long vegetative period precedes the spore- 
formation. The sporangia are no longer alike as in Se/agine//a, but differ 
widely in form and position, and are located on distinct male and female 
strobili. ‘lhe microsporangia, or pollen-sacs, produce after the usual tetrad 
division the microspores, or pollen-grains, which are shed at maturity. 
The male prothallus which they produce is partly formed on the parent 
plant, partly after shedding, and is restricted merely to a few cells (Fig. 28). 
Typically the megasporangia, or ovules, develop each only a single mega- 
spore—or embryo-sac as it is called in Seed-Plants—and within it there is 
at the period of fertilisation a massive female prothallus, bearing archegonia 
(Fig. 27). Since the male and female strobili are distinct, it is necessary 
for fertilisation that the microspores, or pollen-grains, should be shed; 
but no independent vegetative thallus is produced from them: the pollen- 
grain, landing on the apex of the megasporangium, forms a pollen tube or 
siphon, which penetrates the sporangial wall, and by its means the non-motile 
male cells are transferred to the ovum. The essential 
point of fertilisation’ is the same as before, but the 
means are different. The dependence on external 
fluid water, characteristic of all Pteridophytes, is 
dropped, and the siphonogamic method of fertilisation 
may be held to mark the distinctive terrestrial habit. 
But as a lately acquired proof of the justice of 
Hofmeister’s comparisons, the fertilisation by a motile 
spermatozoid is still retained, in a somewhat un- ie es 
practical form, in certain primitive Gymnosperms, Phaddrsccniila ebigintea. 
‘ Pollen-grain, showing the 
Cycadaceae, and Ginkgoaceae. The nursing of the gQ;qnSiaie Some dial 
embryo in the female prothallus, or endosperm, Fc geal clan aie X 540. 
follows in the Pine on essentially similar lines as in 
Selaginella, also the final germination to establish again the independent 
sporophyte. 
Lastly, in the higher Seed-Plants, or Angiosperms, which Palaeontology 
indicates as of later origin, the outline of the life-cycle is as in the 
Gymnosperms, but with still further reduction of the prothallial development 
in the pollen-grain (Fig. 29). Fertilisation is of the terrestrial siphonogamic 
type. The embryo-sac remains like that of Gymnosperms embedded in the 
tissue of the parent plant: it contains before fertilisation only an exiguous 
tissue-development, the exact homology of which is still a question in 
debate (Fig. 30). 
The above sketch illustrates the general trend, though probably not 
the exact course, of evolutionary progress in the Archegoniate series. But 
it is necessary to remark that the examples selected do not form any actual 
phyletic sequence: of them all no two (excepting Lycopodium and Selaginella) 
belong to a single recognised phylum. The general result of their com- 
parison is therefore a history read between the lines. But, with this 
proviso. the following conclusions may be drawn from it, as to the 
