CANNON: Stupies In PLranr Hyprips 145 
species * * * and that of their hybrid offspring, graduates, by an 
almost infinite number of steps, from zero * * * up to complete fertil- 
ity.” Focke (¢. c. 477) gives genera or families that illustrate the 
variation of plants as regards the fertility of their hybrid offspring. 
The hybrids of Papaver, Viola, Verbascum and Digitalis are slightly 
fruitful ; those of Anemone, Nicotiana, Mentha, Crinum, the Cucurbi- 
taceae and the Passifloraceae are often so; while in the following 
there are more fruitful than unfruitful hybrids : Aquilegia, Dianthus, 
Pelargonium, Geum, Epilobium, Fuchsia, Cotyledon, Begonia, Cir- 
stum, Erica, Rhododendron, Calceolaria, Quercus, Salix, Gladiolus, 
Flippeastrum, the Gesneriaceae and the Orchidaceae. While the 
crosses between distinct species produce hybrid progeny that are 
variable as regards the quality of fertility, it seems that nearly 
all hybrids from plants more distantly related are wholly sterile. 
As an example of fertile bigeneric hybrids, however, the well-known 
Aegilops wheat may be cited; but this rule is apparently invari- 
able as regards the few that have parents from different families. 
When the abnormal condition of the male reproductive organs 
causes sterility, as is likely to be the case, this infertility may be 
brought about by a variety of structural causes. The anthers may 
contain pollen but dehiscence may not take place to allow the 
pollen means of escaping, or pollen may be entirely wanting. In 
speaking of this question Focke (/. c. 478) says that “in andern 
Fallen besteht der Bliithenstaub aus kleinen pulverigen bei Anfeuch- 
tung nicht quellenden K6rnern von ungleicher Form und Grosse, 
denen gewonlich einzelne wohlgebildete keimfahige Pollen- 
zellen beigemischt sind.” The conclusion in this case is that 
the abnormal pollen is the cause of sterility and that the normal 
only is functional. Also, according to Duchartre * Naudin made 
the observation that the relative fertility of hybrids was ex rapport 
with the number of normal pollen-grains formed in the hybrid. 
Another and much less usual cause of sterility is found in the 
apparent incapacity of the female organs to be fecundated, but in 
many such cases it may be that there is no morphological defect 
in the ovule. Such a hybrid may sometimes, or usually, be fruit- 
ful if fertilized by another plant of the same strain, or by one of 
the parents, and it is occasionally fertile when _self-fertilized, of 
*Duchartre. Ann. Sci. Nat. Bot. 1V. 19: 129. 1863. 
