REPRODUCTION 447 
of the archegonium, forming the calyptra. The same 
thing may be noted in those Rhodophycee which produce 
a bulky cystocarp. The stimulus is, however, most easily 
observed in the Angiosperms, where it produces effects in 
several regions of both gametophyte and sporophyte. The 
oospore is excited to growth, and after a series of cell- 
divisions becomes the embryo, while the definitive nucleus 
of the embryo-sac similarly inaugurates a series of divi- 
sions, ultimately giving rise to the endosperm, and other 
parts of the ovule undergo modification, so that the seed 
can shortly be recognised. Parts of the flower also exhibit 
renewed growth and further development, the carpels 
especially, though not exclusively, showing an almost 
coincident enlargement, which often attains considerable 
dimensions, so that a bulky structure known as the fruit 
is produced. The new tissue is usually ordinary paren- 
chyma, and in most cases it becomes conspicuously succu- 
lent and frequently strongly acid. The attainment of its 
maximum development is followed by a process technically 
known as ripening. This may take one of two directions ; 
the tissue may become dry and woody, the cells losing 
nearly all their water, and their walls becoming converted 
into lignin. On the other hand the succulence may persist 
and even increase; in such cases the acidity frequently. 
becomes very much diminished and a considerable quan- 
tity of sugar is formed. Other changes in the cells lead 
to the appearance of various flavouring matters, and often 
of substances that are aromatic. Fruits thus acquire 
special characteristics of flavour and fragrance which 
they do not possess while they are young. The chemical 
changes which give rise to these peculiarities are very 
diverse, and cannot be said to be fully understood at 
present. . 
We have noticed that the asexual reproductive cell, 
whether spore or gonidium, is generally found to remain 
in a state of quiescence for some time after its formation. 
The same thing is seen, though not so constantly, in the 
