IN ANGIOSPERMS 43 



system of root and shoot. A long vegetative period precedes the spore- 

 formation. The sporangia are no longer alike as in Selaginella, but differ 

 widely in form and position, and are located on distinct male and female 

 strobili. The 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- FIG 



practical form, in certain primitive Gymnosperms, rradescantia vir&nica. 

 Cycadaceae, and Ginkgoaceae. The nursing of the gBfftiS *Sffik8S 

 embryo in the female prothallus, or endosperm, ( a Afte a rtfasbur V ger e j 1 ' X54 ' 

 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 



