144 CANNON: STUDIES IN PLANT HysrIpDs 
ness, or the reverse of this, the ability to withstand warmer cli- 
mates. Of the former the hybrid between Montbretia Pottsit and 
Tritonia aurea is given by Swingle and Webber (/. c. 414), who also 
present the Le Conte and Kieffer pears as examples of the latter. 
These writers say (2. c. 416) that the ‘adaptability of the Kieffer 
and Le Conte pears to growth in warmer climates is doubtless 
derived from the mother,” that is, the Chinese sand pear. The 
ability to resist diseases is another transmissible character. The 
French varieties of grapes have been made resistant to phylloxera 
by crossing with hardy American forms. The hybrid may also 
be different from either parent in qualities analogous to the above. 
For example, it may be more vigorous than either parent, or may 
attain a larger size. Such variations are found in the offspring of 
closely related plants. The walnut hybrid of Burbank (/uglans 
regia X J. Californica), mentioned by Swingle and Webber, ‘“‘ grows 
twice as fast as the combined growth of both parents. * * * 
The wood is very compact, with lustrous, silky grain, taking a 
beautiful polish, and as the annual layers of growth are an inch 
or more in thickness and the medullary rays prominent, the effect 
is unique.” 
We shall take up now briefly the relative sterility or fertility 
of hybrids, and what is closely associated with it, the limits of a 
cross. ; 
Focke * says that, as a general rule, “je naher die morpholog- 
ische und systematische Verwandtschaft der Stammformen ist, um 
so weniger pflegt das geschlechliche Fortpflanzungsvermogen der 
Mischlinge von der Norm abzuweichen; je ferner die Stammformen 
einander stehen, um so mehr zeigt sich durchschnittlich die Frucht- 
barkeit der Mischlinge geschwacht.” 
Increased or total sterility is brought about in various ways, as 
for example, there may be no pollen and no good seeds produced, 
or there may be a smaller amount of either than in the pure parents. 
Focke says that no peculiarity of hybrids has attracted so much 
attention as the lessening of the power of sexual reproduction, 
and it has been pointed out how important this fact was for- 
merly supposed to be in its relation to the limits of species and 
varieties. Darwin (/. c. 2: 218) says that ‘the sterility of distinct 
* Focke, Die Pflanzen-Mischlinge, 489. 1881. 
