SPECIES CROSSES ES' THE GENUS NICOTIANA. 203 



mother species are entirely indifferent, one arrives at a series 

 of cases in which (1) a slight swelling of the pistil is initiated ; 

 (2) some ovules swell, but develop no embryo ; (3) embryos 

 are produced, but these are incapable of germination ; 



(4) seeds are obtained capable of germination, but the 

 hybrids cannot be reared so as to reach the flowering stage ; 



(5) an increasing number of perfect seeds is formed — very 

 seldom, however, reaching the full number which would 

 have been produced by normal fertilization. Most of these 

 stages are to be observed in the genus Nicotiana. 



Contrasting the combinations which do and those which do 

 not give rise to hybrid offspring, a certain " common sense " 

 relationship seems, so far as my experience goes, to be observ- 

 able. That is to say, in general terms, types between which 

 it is comiiaratively easy to picture an intermediate form before- 

 hand often give rise to hybrids when crossed together ; 

 whereas in cases where an intermediate form is difficult to 

 imagine, it is often apparently impossible to obtain cross- 

 bred offspring. 



So rough an observation would be hardly worth recording, 

 except for the purpose of pointing out that the above assertion 

 is not identical with the common statement that more nearly 

 related species are easier to cross than more remote species. 

 Numerous exceptions undoubtedly occur to both rules, if rules 

 they can be called. 



6.— THE EFFECT OF CONDITIONS UPON THE 

 FORMS OF THE HYBRIDS. 



The effect of external conditions upon the growth and 

 appearance of plants is well known to be often very consider- 

 able, but the full extent of this effect is seldom realized. It is 

 highly imr)ortant to get an idea of the amount of change which 

 can be induced in this way before one begms to base any 

 conclusion upon the comparison of individual plants with one 

 another. Plate 18 shows two F i individuals of Nicotiana 

 tabacum X glauca. The plants represented were grown from 

 seeds sown at the same time and reared under approximately 



7(1)09 (27) ^ 



