438 VEGETABLE PHYSIOLOGY 
and which leads to the production of the structure known 
as the seed, the latter being a special body produced by all 
members of the group of Spermophytes or flowering plants, 
and now marking them off clearly from all below them. The 
phenomenon in question is known as heterospory. Plants 
which exhibit it bear two kinds of spore, which differ from 
each other mainly in their relative dimensions. Some 
are produced in large numbers in a sporangium and have 
usually the structure which has already been described. 
Others are much larger than these, and are developed either 
singly or in small numbers, usually four in a sporangium. 
They are spoken of as microspores and megaspores respec- 
tively. In the Pteridophytes the megaspores, when formed, 
differ from the microspores chiefly in size; in the Spermo- 
phytes they are never liberated from the sporangium and 
have consequently thin and 
delicate walls. 
The phenomenon of 
heterospory involves the 
production of two gameto- 
phytes to one sporophyte, 
as each of the spores pro- 
duces its appropriate pro- 
thallium. The gameto- 
phyte arising from the 
Fie, 175.—GrRMinatioN OF A MASS CF 
Microspores or Salvinia. (After Miele aa eb Pe only 
Sachs.) to male gametes, that from 
\. The mass protruding tubular prothalli th 
from different spores; 2, a prothallus ow aeons only to 
more highly magnified, showing an female.ones. Such plants 
antheridium, «@; 38, antherozoids in ‘ . A 
mother-cells; 4, ruptured antheridium, show in their life cycle, 
therefore, three forms, one 
sporophyte and two gametophytes, the latter occurring 
synchronously. 
The male gametes are freo-swimming antherozoids in all 
Pteridophytes and are developed in antheridia of varying 
structure. The females are oospheres, produced in archegonia. 
The gradual appearance or development of the seed can 
