398 TRANS. ST. LOUIS ACAD. SCIEN'CE. 



tested have well germinated ; in fact, as fax- as I know, no differ- 

 ence in fertility or germinating power between them and the 

 acknowledged species has been discovered. The seedlings of 

 such questionable individuals do not seem to revert to a supposed 

 parent, a sport of which they might be claimed to be, but propa- 

 gate the individual peculiarities of the parent ; " come true," as 

 the nurserymen express it. For how many generations this may 

 continue, and whether in time forms approaching one or the other 

 parent may not ap^^ear, remains to be seen. At the same time it 

 is a remarkable fact, that, notwithstanding their fertility, they do 

 not seem to propagate in their native woods ; we may properly 

 ascribe this to a lesser degree of vitality in the hybrid progeny, 

 which causes them to be crowded out in the struggle for existence : 

 one of the provisions of nature to keep the species distinct ; or, as 

 Dr. Gray suggests, fertilization by one of the parents may soon 

 extinguish the hybrid characters. I find ten forms, enumerated 

 below, which I consider as real hybrids ; of them only a few, often 

 only single individuals, have become known. Their existence 

 cannot well, without straining facts, be considered due to innate 

 variability in the supposed parents. When more carefully looked 

 for, undoubtedly more will be discovered. 



White-oaks and Black-oaks are too distinct to be crossed. 



Among the White-oaks hybrids seem to be much rarer than 

 among the Black-oaks, or it may be that they are more difficult 

 to discover. Of the former, I have thus far been able to trace 

 3 forms only which I must take for hybrids, and all of them point 

 to ^. alba as one of the parents. 



^. alba X macrocarpa is sent by M> S. Bebb (No. 27) from Northern Illi- 

 nois ; the leaf is that of alba, with a persisting down on the under side ; the 

 cup is not larger than in alba, but a little deeper and with the prominent 

 triangular scales oi macrocarfa. Another specimen, much nearer macro- 

 carpa, was studied by E. Hall, a number of years ago, in central Illinois 

 (Amer. Ent. & Bot. 1870, p. 191). The leaves are those of the latter species ; 

 the bark, the down of the young leaf and inflorescence, and the acorn, more 

 that of alba; while the deep cup, covering half the acorn, is that of a small 

 fruited macrocarfa, but entirely destitute of a fringe; its acorns develope 

 much slower than those oi macrocarf>a, and more conform to alba. Of the 

 allied oaks, macrocarpa, alba and stellata, the first developes its acorns 

 soonest and the last latest; these last are not larger than peas by the end 

 of August, when those oi alba are nearly full grown. 



