78o LIFE : OUTLINES OF GENERAL BIOLOGY 



This cone bears stalked sporangia, but apparently no sporophylls. 

 Modern types are homosporous, and the outermost layer of the 

 spore cracks into two peculiar coiled bands, which unwind and get 

 entangled with their neighbours. There are usually separate male 

 and female gametophytes, the latter the larger, forming small green 

 branching thalli on the surface of the soil. The egg-cell in the arche- 

 gonium is fertilised by a large biciliate sperm-cell, produced in an 

 antheridium; and the result is the familiar sporophyte. The schema 

 runs: 



Spores >■ Above-ground > P>rtilised > Sporophyte with 



green gametophyte egg-cell sporangia on 



usually dioecious a sort of cone 



With interesting resemblances to the Lycopods, but oftenest 

 ranked with Ferns, are the aberrant Adder's Tongue (Ophioglossum) 

 and Moon wort (Botrychium), in both of which the "fertile spike" 

 of sporangia is very distinct from the vegetative part of the 

 sporophyte. 



ORDINARY FERNS. — The highly successful fern group goes 

 back to the Devonian, and is represented by 3,000 or more living 

 species. They are probably the only plants whose "alternation of 

 generations" has been noticed by casual observers, for it is very 

 well known that what grows from the sown spore of a fern is a very 

 different kind of plant, a little, fiat, green disc on the surface of the 

 damp soil. From this the beginning of the asexual spore-bearing 

 plant can often be seen growing. 



From a spreading underground stem, the leaves or fronds arise, 

 sometimes, as in Bracken and Royal Ferns, to the height of a 

 man. On the back of a leaf are numerous sporangia, sometimes with 

 an enormous total, though with a small individual production of 

 spores. On a damp substratum a spore develops, through a fila- 

 mentous phase, into a green prothallus, with white rhizoids below 

 and also the sex organs. The antheridia are usually on the older 

 ventral part among the rhizoids ; the archegonia are later of appear- 

 ing, and are situated near the apical notch. The large sperm has 

 numerous cilia, and it is interesting to find that its nucleus is usually 

 the only part that actually enters the egg-cell. From the fertilised 

 ovum in the archegonium the familiar sporophyte develops. The 

 schema is: 



Spore > Prothallus > Fertilised > Sporophyte with sporangia, 



with egg-cell usually homosporous, 



antheridia heterosporous in the 



and archegonia water-ferns 



Sometimes a sporophyte develops from a prothallus without 

 there being any fertiHsation. This "apogamy" may imply either a 



