282 
F. Cavers. 
mentioned, there is no perianth, and the same is true of practically 
all the forms with a well-developed marsupium. 
In a considerable number of genera, belonging to different 
families, we find a series of changes which lead up to the formation 
of a distinctly sac-like structure, arising after fertilisation from the 
tissue of the stem and resulting in the development of a definite 
and'conspicuous marsupium. Three main types may be distinguished. 
