20 BOTANICAL GAZETTE [JANUARY 
they found abortion in all of the sporangia in C. Binneyana and in 
only one-fourth of the sporangia in C. Casheana, they suggested that 
the abortion of certain of the spores and the consequent increased 
nutrition of their surviving fellows may have been the physiological 
condition that ultimately rendered possible the development of mega- 
spores. 
In 1908 THopay (27) added further and convincing evidence from 
Sphenophyllum Dawsonii, to that already furnished by WILLIAMSON 
and Scott (28). He shows that the size of the spore varies with the 
amount of abortion, and that while the average diameter of the spores. 
in this form is 83 , the maximum diameter where abortion is extensive 
is as much as 120#, or nearly one-third larger. These are both 
striking examples of heterospory as it must have appeared in its 
incipiency, and both show a very definite relation between the size 
of the spore and the extent of abortion. 
Statement 
Many have insisted that the only clue furnished as to the possible 
origin of heterospory is that found in Equisetum, where the prothallia 
are usually dioecious. This dioecism seems to be due rather to exter- 
nal than to internal conditions, as shown by HoFrMEISTER (13) aS 
early as 1855, when he pointed out that while the prothallia were for 
the most part dioecious, they could be made to produce either male or 
female sex organs, or both, by varying the external conditions. He 
states that archegonia may appear on late shoots of the so-called male 
prothallia; while SADEBECK (23) shows that antheridia may appear 
at a later period on the lobes of the female prothallia. 
GOEBEL (12) says that “it is probable that in this, as in other 
cases, the male prothallia are those which have been insufficiently 
fed.” Bower (2) calls attention to the fact that the spores of Equise- 
tum show no differentiation in size, or apparently in sex. CAMPBELL 
(3) also states (p. 453) that “external conditions influence the pro- 
duction of males or females as in the ferns, and that while the prothal- 
lia are normally. dioecious, this is not exclusively the case.’’ I have 
examined critically several laboratory cultures of the prothallia of 
Equisetum in which only antheridia were produced in the crowded 
portions of the sowing. In fact I have grown a number of cultures 
