446 VEGETABLE PHYSIOLOGY 



thing may be noted in those Khodophyceae 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 

 case of the zygote. In the Thallophytes this resting period 



