MINUTE STRUCTURE OF PLANT HYBRIDS. 275 



(d) On the Divergence of some Hybrids, or Parts of Hybrids, toivards One Parent. 

 — It is undoubted that not a few hybrids show a decided leaning towards one parent, 

 though I consider from examination of several that have thus been described that the 

 number has been considerably over-estimated. With undoubted cases we will now 

 concern ourselves. Eegarding these many have asserted that the male parent or male 

 element predominates, and sets up one-sided variation changes. We would readily grant 

 both from perusal of Focke's " Pflanzen-mischlinge " and from direct observation that 

 this is frequently true. But no one can deny that there are many artificial hybrids 

 which do take more after the female parent. Professor Brookes,* recognising the 

 strength of the former position, has formulated a theory of variation and heredity alike 

 ingenious and plausible. In the present state of our knowledge we would not reject it, 

 but we may still inquire whether some other and simpler explanation cannot be given. 



If we view the male and female sex elements of any plant as aggregations of a 

 purely physical but very complex set of substances, it must necessarily follow that if the 

 relative amount, or weight, or combination proportions, in each male or female element 

 varies, variation will result after conjugation, and it will only be where the amount in 

 each conjugating cell is an exact average of the producing organism that a new 

 organism will develop which will show throughout an average combination of the 

 characters of both parents. Now in the struggle for existence which holds among pollen 

 cells and egg cells it will seldom happen that exactly the same amount of sex substance 

 will be formed in exactly similar combinations in each. But of the two, which, we may 

 ask, will vary most ? Direct observation proves that it is the male or pollen cell, and 

 the reason for this is obvious. The great majority of flowering plants mature their 

 ovules with the contained egg cells inside cavities where space for growth, and elaborate 

 means for nutrition and protection up to the time of fertilization exists. But the 

 opposite is the case with pollen cells, which are crowded together in the anther cavity, 

 and often obtain nourishment by approximate sustenance through each other. Those 

 therefore nearest the sustentative source will have the advantage, unless of course they 

 are strongly pressed against by some firm bounding wall. 



I have been greatly surprised both with the average constancy in size of the 

 egg cell and with the greater variability of the pollen cells in such plants as Lilium, 

 Scilla, and Digitalis. But we now know that the nuclear substance is specially con- 

 cerned in fertilization, and Strasburger has formulated a hypothesis t to account for 

 diversities in hybrids by supposing that the two parents have a different average amount 

 of chromatin substance in their sperm and egg nuclei. We would extend the hypothesis 

 further, and regard the amount as a varying one even in the same parent. Hitherto I 

 have not been able to measure the nuclei of sex cells, but in many vegetative cells there is 

 clear evidence that the variability in size of the nucleus is very great, and further that 

 there are considerable differences in the size of the nuclei and nucleoli even in adjoining 

 cells. So much is this the case that I have felt quite safe only in comparing the 



* The Laio of Heredity, Baltimore, 1885. t Neue Untersuchungen, p. 163. 



VOL. XXXVII. PART I. (NO. 14). 2 R 



