158 CANNON: STUDIES IN PLANT HysBrips 
Generations LS IG Ils IIc 
As a gf and 9— Pts POA carte a et 
| ne EEE ay eee ine a 
Miwseovnedisgetacscsce’s Aa raf x ae i \ etc 
| x ie eae A 
, a Na and: Oo — Py Fee TOUSEN Ae ele 
Fic. 2. Hybrid homosporous monoecious fern. Explanation. .S, sporophyte ; 
G, gametophyte; 4 and a, the chromosomes of the two races, when combined (4 a) 
the form is hybrid. 
Generations pe IG ILS iG 
Ass a OF: Oa 7 es 
see perma y went Aa “xia Zt 
; oe x°*2i¢ 
a Na aor 0 a 
Fic. 3. Hybrid homosporous dioecious fern. Lettered as the preceding figure. 
In contrasting the two formulae for fern hybrids as given 
several points may be worth speaking of. In the first place the 
fern hybrids with monoecious prothallia revert after the Prsum- 
type of Mendel; and secondly the hybrids with dioecious sexual 
generations are always some form of intermediate, and never 
revert. Dioecious forms also, such as Lguisetum hybrids, sug- 
gest problems of interest not necessarily closely connected with 
their hybridity. Among these the following may be mentioned: 
Dioecious ferns viewed from the standpoint of the purity of the 
spores, and hence of the sex cells, give suggestions as to the 
morphological basis of the origin of heterospory. Coulter * says: 
“The evolution of heterospory seems simple enough. The physio- 
logical differentiation of the spores was complete when prothallia 
became persistently dioecious. * * * A prothallium producing both 
sex organs equally well may be regarded as in a state of equilibrium, 
an equilibrium which is disturbed by any conditions which favor 
the production of one sex organ rather than the other, in this 
case probably nutritive conditions. This disturbance of the equi- 
librium of a bisexual prothallium would certainly find an expression 
first in a dioecious tendency, and finally in a dioecious habit. With 
the habit once fixed the morphological differentiation of spores 
becomes inevitable, since the nutritive requirements of the two 
prothallia are so different. The evolution of heterospory seems to 
be one of the simplest of selective processes, with inequalities of 
nutrition to furnish the variations.”’ 
* Coulter. Bot. Gaz. 26: 163. 1898. 
