REPRODUCTION 457 
cause not only the development of the sporophyte from 
the oosphere, but a considerable additional growth of the 
archegonium, forming the calyptra. The same thing may be 
noted in those Rhodophycew which produce a bulky cysto- 
carp. 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 divisions, 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 parenchyma, and in most cases it becomes conspicu- 
ously succulent and frequently strongly acid. The attain- 
ment 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 con- 
verted into lignin. On the other hand the succulence may 
persist and even increase; in such cases the acidity fre- 
quently becomes very much diminished and a considerable 
quantity 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 
