I VI KODUCTION. 



i:; 



two species whose flowers are of different size, those of 

 the hybrid are frequently of thi e or approxi- 



mate I of the bloom of the Bpecies having the 



larger Examples of uncommonly large flowers 



i'!i in Dianfhus a < D. superbus, Rubin 



■•'■ /,'. bellardii, hybrids of Rosa gallica, Begonia 

 boliviensis and Isoloma tydceum. 



A high vegetative power is very conn i in hybrids, 



as in Nymphaa, Rubus ccesius, Nicotiana suaveolens X 

 itissima, Linaria striata > /.. vulgaris and Potamo- 

 geton. A greater duration of life has been noted in i on 

 in. M. .n with several hybrids»of Nicotiana and Digitalis. 

 An increased resistance to cold has been noted espi ciall 

 in Nicotiana ■ ns > N. tabacum latiss.; while, on 



tl ther hand, Salix viminalisX, S. purpurea is more 



sensitive to cold than either parent species. 



These facts point in part to an apparei :ned 



vitality of hybrids in consequence of their abnormal mode 

 of production ; and in part in some instances to an i 

 ordinary vegetative power. The cause of this last phe- 

 nomenon, which is observed less frequently than les 

 vitality, has been in some degree only recently under- 

 stood. Noteworthy experiments of Knight, Lecoq, and 

 others have been published, hut it, has been through the 



taking re of ( lharli - I 'arwin thai thi 



with which a cross between different individuals and 

 i i ■ .: ... i ml the same spi cies is effected was first 

 clearly explained. The increase of the vegetative pi 

 in hybrids is clearly a phenomenon thai closely i 

 ds with the peculiar conditions of hybrid produ 



and in-' .Is not a special explanation. It was at lii I 

 thought that lessened fertility was compensated for by 

 greater vegetative luxuriance, an hypothesis that Uiirt- 

 ner has shown to be untenable, as is evident by the fad 



nany of the most fertile hybrids (Durata, MirabUis) 

 are a ble for the largest growth. 



1. Partial on Complete Sterility of Hybrids. 



Sul rmal fertility of hybrids, especially as regards 



the pollen, has long been recogni ed as one of the mosi 

 important criteria of hybrids. It seems, however, that 

 . baracter like intermediateness has been an almost 

 unbridled conception and hence greatly overvalued as a 

 distinguishing feature. Focke in his summary gi 

 a weall b of fa conni ct ion : 



I'm RTII PBOPOSITION. 



Hybrids between different species shntr in their anthers a 

 smaller number of normal pollen-grains <in<l a smaller 

 number of normal seed than in plants of pure descent. 

 Frequently they produce neither pollen nor seed. In 

 hybridi o • related races <>• s weakening 



of the poirer of sexual reproduction is not present. 



rili or nearly sterile hybrids usually remain 

 li for a long lime. 



No property of hybrids has attracted so much at! 

 tion as the les of the ability of sexual reproduc- 



tion. Kolreuter believes that this peculiarity permits 

 a sharp border-line to be drawn between species and 

 hen many botanists h&\ 

 view, ami lately B. Naudin, Decaisne, and Caspary 

 adopted it in a more or less modified form. K 

 and Klotzsch, and before them Godron, hold that the 

 pollen of hybrids is entirely impotent, which contention 



had already been disproved by Kolreuter' 

 scan he-. Kolreuter is ■ I with the promulga- 



tion of the doctrine of complete Bterility of hybrids, hut 

 this erroneous chai : only tin 



an ignorani r misu 



Kolreuter docs not dily, hut 



of a I'- .in d fei : 1 1 ' . ■ iversal property of hj ; 



In different plant genera the fertility of hybrids 1- 

 very varied. Fertility is observed in a very low di 

 in the hybrids Papax '■ I um, and Digit 



it is i 'I in .1 1" inline. .'. Mentha, 



i ' rin a in , < ' in nriii I in hi, anil I'n iflorat ea ; and it is more 

 common I I ity in A q 



niii in , '.. a in , Epilobiun I don, Bi g 



Cirsium, Erica, Rhododendron, I 

 Salix, Gladiolus, Cypripedium, and Hip} In 



nera Vitis, Prunus, Fragaria, and Pirus, hybrid- of 

 i lic.lv related are used .as seed-bearin 



and in Cereus the hybrids of widely separated speciea 

 -how undiminished fertility. 



The sterility of hybrids is expressed at times by their 

 showing no inclination to flower, which peculiarity has 

 been >■ specially in several hybrids of Rhododen- 



dron, Epilobium, Cereus, and Hymenocallis ; but I 



i epl ions, inasmuch as hybrids u 

 abundantly and earlier than true spi i 



In hybrids with unisexual flo 

 fall off « hen in the hud. as in Cu A Bt - 



gonia (hybrids of B. frwbeli \. DC). In bisexual flo 

 the stamens arc stunted, as noted in several hybri 

 Pelarg and Digitalis (/'. lutea~XD. purpur 



tubiflora Lindl.). The mosi common -.quel of h; 

 product ion is a deficienl developmenl i Hen-grains 



in hybrid plants. Commonly the anthers of 1;. 

 sterile and do not contain any pollen; or they are 

 small and do not - ' ieney of poll- 



noted in Rubus idceus X R- odoratus, Ribes aureum X 

 /,'. sanguineum, and Alopecurus genicvlatus X A., pra- 

 In other cases the stamens produce small pow- 

 dery grains which do not swell with moisture, which are 

 of varying size and shape, and with which are usually 

 mixed a few single, well-formed, embryo-forming pollen 

 3. Tic number of normal grains is. however, fre- 

 quently larger, and comprises LO, V 11 . or more per 

 of the total number. Large, rough grains which swell 

 with moisture. all well-formed grains, 



a iv present often in greater or less number anion? the 

 stunted grains. In hybrid- of closely related spec' 

 in Melandryum album X M. rubrum, hut little irregu- 

 larity is usually found in the form of the pollen-irrains. 

 in one hybrid, Sinningia, the pollen was better in the 

 1 year of flowering than in the first. 



In the hybrids of unquestionably different speci 

 normal formation of the stamens is seldom met with. 

 Assertions in support of this still need confirmation, in 

 part, therefore 1 refer to Nymphaa lofiis X -V. rubra. 

 ■riii rubrovenia X B. xanthina, Isoloma ty 

 ■ purpurea X 8. 

 grains which are all of nearly the same form are f 

 in Salix aurita, and >'. caprea and S. viminalis X S. 

 ns. 



On lopment of 



trs less frequently in race crossings. Possibly, fur- 



