REPRODUCTION 429 
them, 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 Phanerogams or flowering 
plants, and 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 Phanero- 
gams they are never liberated from the sporangium and 
have consequently thin and delicate walls. 
The phenomenon of heterospory involves the production 
of two gametophytes to one sporophyte, as each of the 
spores produces its appropriate prothallium. The gameto- 
phyte arising from the 
microspore gives rise only 
to male gametes, that from 
the megaspore only to 
female ones. Such plants 
show in their life cycle, 
therefore, three forms, one 
sporophyte and two game- 
tophytes, the latter occur- 
ring synchronously. 
The male gametes are gyo, 175—Gurwination of A MASS oF 
free swimming anthero- Microspores or S«alvinia. (After 
Sachs.) 
zoids in all Pteridophytes 1, The mass protruding tubular prothalli 
* from different spores; 2, a prothallus 
and are developed iM an- more highly magnified, showing an 
sa: s "T1R- antheridium, @; 38, antherozoids in 
theridia of yale struc mother-cells; 4, ruptured antheridium. 
ture. The females are 
oospheres, produced in archegonia. 
‘The gradual appearance or development of the seed can 
