HYBRIDS. 455 



1173. Nectar is protected in various ways from unwelcome 

 insects ; that is, from those which cannot aid cross-fertilization. 

 The chief of these is bj' the structure of the flower itself or the 

 parts below. Characteristic odors and certain colors maj- con- 

 tribute to this protection. Thus, as MuUer has pointed out, 

 dull yellow flowers are entirely-, or almost entirely, avoided by 

 beetles, while the^- are visited by Diptcra and Hymenoptera 

 (flies and bees). 



1174. Hybrids are the offspring of crossed species. But, as 

 shown in Volume I. page 320, the limits which separate varie- 

 ties from species are sometimes not sharply defined ; hence it 

 happens that the term hybrid has been also a[)plied to crosses 

 between stronglj- marked varieties of the same species. Such 

 offspring should, however, be termed either variety-hybrids or 

 cross-breeds, and the word hybrid kept to its proper significa- 

 tion. 



1175. Wide differences exist in the degrees of capacitj- for 

 producing hybrids. Thus certain closely allied species cannot 

 be made to cross, while others much more remote in apparent 

 relationship are crossed without difficult}-. 



1176. In general the limits of capacity for hybridizing do not 

 extend be3-ond the genus ; a few cases, however, are known in 

 which species usually assigned to different genera have been 

 successfully crossed.^ Hence it cannot be known beforehand 

 whether the attempt to cross two species will be successful. 



(3) Flowers which are always open, but which are odoriferous at one time 

 and scentless at another. Under this class there are also two subdivisions : 



(a) Those always open, and only odoriferous during the day ; c. g. 

 Cestnim diurnum, Coronilla glauca, etc. 



(i) Those always open, and only odoriferous at night ; e. g. Oestrum 

 nocturnuni, Hesperis tristis, etc. 



In certain eases odors are given out by flowers in an intermittent manner. 

 This is strikingly shown in some of the larger night-fiowering species of 

 Cactaceae. 



Deliiino has given (Ulteriori Osservazioni sulla Dicogamia nel Regno Vege- 

 tale, 1868-1874) an elaborate classification of odors as they exist in Howers. 

 He makes forty-five kinds which are readily distinguishable as peculiar, while 

 bet'.ieen these kinds there are of course innumerable gradations. 



1 Focke notes that hybrids between species belonging to different genera 

 are comparatively common in the following families : Caryophyllaceie, Melas- 

 tomacea), PassifloraccEe, Cactaceae, Gesneriaceae, Orchidaeeffi, Amaiyllidaceffi, 

 and Gramines; and he cites also tlie following instances outside of these 

 families ; Brassica X Raphauus, Galium X Asperula, Centropogon X Sipho- 

 campylus, Campanula X Phyteuma, Verbascum X Celsia, Philesia X Lapageria. 

 (Pfianzen-mischlinge, 1881, p. 456). 



