232 
RUTH HOLDEN 
the case in birch crosses, the Fi is very variable, so that some individuals 
are like one parent, and others Hke the other; or some, instead of being 
members of the first filial generation, are the result of long continued 
crossing and back crossing. The first explanation has one objection 
— why should E. trachyodon always be produced on the other side of 
the water, and E. variegatuni var. Jesupi on this? Another difficulty 
is our ignorance of the way in which these infertile hybrids reproduce 
themselves. Are some of the spores capable of germination, or does 
the crossing have to take place anew for each individual? Although 
certain of the large spores are apparently normal, Duval Jouve, who 
had no difficulty in artificially cultivating other species of Equisetum, 
was unable to do so in the case of E. littorale. Furthermore, Milde has 
called attention to the fact that these supposed hybrids always grow 
in the vicinity of the parents.^ 
None of these points can, be settled conclusively at the present time. 
The facts indicate that E. variegatum var. Jesupi is a hybrid between 
E. variegatum and E. hiemale, but until the cross is actually made under 
artificial conditions, the exact relation must remain uncertain. The 
genus Equisetum will be however a particularly favorable one for ex- 
perimental work, since the various species are distinguished by a 
comparatively large number of clean cut and constant anatomical 
characters. Whether they are dependent on unit factors which are 
capable of independent segregation and recombination will be an 
extremely interesting and important subject for investigation. 
Summary. 
1. Equisetum variegatum var. Jesupi is probably a hybrid between 
E. variegatum and E. hiemale for the following reasons: (i) It has some 
of the characters £. variegatum, some of those of E. hiemale, and some 
which are intermediate between the two. (2) A large number of its 
spores are always abortive. 
2. Whether it represents the immediate offspring (Fi) of the cross, 
its relation to other varieties also intermediate between E. variegatum 
and it. hiemile, whether the cross is repeated for each individual, how 
far hybridization will explain the large number of transitional forms in 
^ A third explanation would be that instead of the result of crossing two species, 
three or even four may be implicated. Brainerd (Rhodora, March 1906) found 
indications of such complications in his work on violets, and cites one, Viola (hirta X 
oiorata) X V. collina, from the Tyrol. 
