488 Davis . — The Origin of the Archegonium. 
of the first sort are very significant, those of the second would 
be readily understood in relation to them. 
Sterilization of reproductive tissue is a well-known tendency 
among plants. It results in the sacrifice of certain reproduc- 
tive cells or tissues, either in relation to environmental condi- 
tions, or through the conservation of food-material by which 
certain cells are favoured in their nourishment at the expense 
of others. The latter condition is illustrated very extensively 
in the asexual reproductive tissue of the sporophyte, and 
among sexual cells notably by the sacrifice of the nuclei 
in the oocysts of the Fucaceae (e.g. Pelvetia ), and during 
oogenesis in certain Phycomycetes (e.g. Albugo , Peronospora , 
&c.). It is of course a fundamental principle in oogenesis 
among animals. If, as seems very probable, the canal-cells 
in the archegonium are degenerate gamete mother-cells, this 
principle finds an admirable illustration, for they are sacrificed 
with obvious advantage to the egg at the bottom of the 
structure, not only for its nourishment but also in relation 
to the mechanics by which the neck of the archegonium is 
opened and the sperms brought to the egg. 
Sterilization of reproductive tissue in relation to environ- 
mental conditions implies such changes as are obviously a 
response to external factors. They are frequently involved at 
the same time with the conservation of food, but this is of 
secondary importance. The most powerful external factor 
affecting an organ is the medium in which it lies. If this be 
air the structure must provide itself with effective protective 
coverings, for the drying action of the atmosphere is perhaps 
the most serious difficulty with which the land plant contends. 
Desiccation must have been the chief danger that aquatic 
plants faced when they left the water, and very little advance 
in internal structure could have been possible until this 
problem was solved by the development of suitable external 
coverings. 
Now let us consider what would happen to plurilocular 
gametangia of aquatic Thallophytes if such forms should 
gradually adopt terrestrial habits. The general protection 
