276 DR J. M. MACFARLANE ON THE 



nuclei of Cytisus Adami with those of its parents. Now if this be true of vegetative 

 cell nuclei, there is strong probability that it will equally hold with reproductive nuclei, 

 and accordingly the greater resemblance of any resultant embryo to one parent over 

 another would be satisfactorily explained on a physical basis. 



But this does not on first look enable us to explain those cases where local 

 divergences toward either parent occur in a hybrid which otherwise is very evenly 

 balanced. Many examples of this have cropped up in the course of investigation and 

 description. But an application of the same hypothesis in its minuter bearings will clear 

 away most difficulties. If we view a fertilized egg of any plant which is about to 

 segment to form an embryo as being not merely a chemically complex nucleated mass of 

 protoplasm, but as a microcosm in which the orderly-arranged molecules of the conjugated 

 male element have so exactly fitted into and become united with corresponding molecules 

 of the female element that after conjugation co-ordinated groups of molecules are set 

 apart as stem-producers, root-producers, leaf-producers, and hair-producers, we will have 

 done much to clear away obstacles. But physically there is no reason why we may not 

 assume that each cell of the future ]3lant has representative molecules in the apparently 

 simple egg. Now if such be the case it may not unfrequently happen that corresponding 

 groups of molecules from male and female cells do not unite exactly owing to incomplete 

 nutrition or other defect in the maturing of one group. Thus one of the two may in 

 part break down or become weakened and the complemental sexual part thereby give to 

 the resulting tissue a one-sided character. Since the nuclear substance of the male or 

 pollen cell is the one most liable to variable development through over- or under- 

 nutrition, or through advantageous or disadvantageous position, it follows that variation 

 will oftenest have its expression from the male side. 



(e) Mechanical or Physiological Obstacles to Fertilization as an Explanation of 

 Infertility in some Hybrids. — Great importance has been attached by many to the fact 

 that some parent species which ajDpear even to be nearly related refuse to cross, or only 

 do so on one side, reciprocal crosses being apparently impossible. But as Strasburger 

 has well emphasized '* a very simple mechanical explanation like that advanced on the 

 animal side by Pfluger in the case of Amphibians may explain the difficulty. Thus it 

 is possible that the sperm nucleus, or pollen tube containing such, of some plant 

 species may be too large for the receptive area of the egg cell or ovular surface, though 

 the opposite application might prove quite fertile. Similarly the relative length and 

 shape of style, size of pollen grain, strength of pollen coat, amount of mucilage secreted 

 by the stigma, time of ripening of stamens and stigma, must all be studied before we 

 pronounce any attempted hybrid union impossible. Equally simple physiological 

 obstacles connected with colour or some special chemical production may help in 

 explaining partial or entire sterility. When treating of Cytisus Adami we noted a great 

 abundance of tannin material in the " purpureus " parent, a relatively small quantity in 

 the " laburnum " parent, and an intermediate amount in the hybrid. This extended even 



* Neue Untersuchuiujcn, p. 194. 



