642 PHYSIOLOGY. 



Hybrid Plants. When species are crossed, the result from the hybrid 

 seed is a plant differing from both parents, bearing more or less relation 

 to one or the other, as regards form and habit, in different cases. 

 Gardeners do not appear agreed as to the kind of influence exerted by 

 the male and female parents respectively in determining the charac- 

 ter of the mule. Gaertner states that in hybrids of Digitalis the mules 

 most resembled the female parent, while in Nicotiana the reverse 

 appeared j and he believes no law can be laid down in regard to this 

 point. 



The seeds ripened after hybridizing generally form but a fraction of 

 those matured under natural circumstances. Thus, according to Gaertner, 

 hybrids of Verbascmn Lychnitis with V. nigrum gave but 63 per cent. 

 of the normal number, with V. Thapsus 21 per cent., with V. pyraini- 

 datum 3 per cent. ; hybrids of Dianthus barbatus with D. Armeria 53 per 

 cent., with D. deltoidcs 22 per cent., with D. virgineus 1 per cent., &c. 

 Darwin, however, shows that the sterility of crossed species as well as 

 that of their offspring varies from zero to complete fertility. Moreover 

 dioacious plants appear less prone to hybridize than those with herma- 

 phrodite flowers. 



The seeds originating from a process of hybridization produce plants 

 varying very much in their degrees of fertility. It appears that the mar- 

 jority are barren ; in many cases only a portion of the seeds formed pro- 

 duce fertile plants ; while in a few cases the hybrid plants are nearly as 

 fertile as those of their parent species when unmixed. Some species can 

 be crossed readily, but the hybrids resulting are very sterile ; on the other - 

 hand, some plants crossed with difficulty yield very fertile offspring. 

 The degree of sterility differs in two species when reciprocally crossed. 

 It is observed, also, that in fertile hybrid plants the flowers earliest opened 

 are the most fertile, or sometimes are the only ones that ripen seed, sub- 

 sequent flowers often developing fruits the seed of which are destitute 

 of an embryo. 



This barrenness of the later flowers, from deficient vital force, is in 

 some degree analogous to what we sometimes observe in cut flowering 

 stems of succulent plants. We have seen the ovary swell and one or 

 more seeds become extensively developed on a cut spike of Aloe and on 

 an umbel of Crinum, thrown aside to wither. The seeds, however, were 

 quite "blind," the expansion consisting of an abnormal development of 

 the integuments of the ovule, the nucleus and embryo-sac remaining un- 

 changed. 



Relative Fertility of Hybrids. In some fertile hybrids it is observed that 

 their progeny forming the second and third generations become more fer- 

 tile than^the original hybrid ; it is noticed, however, that their descendants 

 usually exhibit a great tendency to vary in external character, and often 

 return more or less to the type of one of the parents. 



Hybrid characters seem, from the researches of Naudin, not to be 

 permanent. The plants revert, as above said, to the type of one or 

 the other of the parents. What are termed "sports" by gardeners, i. e. 

 shoots differing in character from those on the other portions of the 

 plant, are frequently, but not always, due to the dissociation of hybrid 

 or metis characters', or they may present the characters of some 'more 

 remote ancestor. 



