MINUTE STRUCTURE OF PLANT HYBRIDS. 205 



tural deviations, which, being perpetuated and exaggerated it may be in size, will ulti- 

 mately appeal to the naked eye. It was this, well illustrated in the group Cirripedia, 

 which forced Darwin slowly but surely to frame and enunciate his evolution hypothesis. 



As plant after plant has passed under my observation, I have been greatly impressed, 

 not only with the average similarity in development that each shows, but even more 

 with the constant tendency there is for individuals to vary from that average either in 

 under or over development, it may be only of some small part or area, or of some large 

 organ. As illustrations on a somewhat large scale, I may refer to the number, position 

 on the stem, and size of leaves, a line of inquiry which has been entirely overlooked by 

 systematists, but which can afford characters of considerable value. Thus Hedychium 

 Gardnerianum, when well grown and not overcrowded in a hot-house, sends up flowering 

 shoots which bear on the average thirteen lamina-producing leaves, beside one or two basal 

 scales. H. coronarium bears twenty-one, while the hybrid, H. Sadlerianum, bears seven- 

 teen. But not unfrequently from overcrowding, lack of light and nourishment, or other un- 

 favourable surroundings, the number in each may be considerably reduced. Conversely, when 

 very favourable vegetative conditions occur, these are accompanied with greater luxuriance. 



A shoot of Saxifraga Aizoon, with freedom for growth, produces annually twenty- 

 three to twenty-six leaves ; S. Geum, forty to forty-five ; and their hybrid, S. Andrewsii, 

 thirty to thirty-two. 



During the autumn of 1890 I happened to go over a large bed of sunflowers, and in 

 by far the greater number twenty-seven to twenty-eight leaves were formed between the 

 cotyledons and terminal capitulum. A few instructive cases of variability from the 

 average were noted. The bed was one which sloped to the sun, and some plants at the 

 back that were slightly overshadowed by trees had been starved in their light and moisture 

 supply. Their leaves were reduced to twenty or twenty-one. On the other hand, one 

 in a favourable situation produced thirty-one leaves. 



But minute changes are correlated with these grosser variations, such as an increase or 

 decrease in the stomata over a given area, or in the length and number of hairs, &c. In 

 the choice of material, therefore, for hybrid investigation one should either be acquainted 

 with the parent individuals and the conditions under which they were grown, or try to 

 choose an average specimen of each for study. 



(b) Limit of Variability. — A wide field for patient and laborious work is open in the 

 direction of ascertaining how far the individuals of a species may differ microscopically 

 without losing specific identity. As yet this field may be said to be untrodden,* but if we 

 are to get an exact estimate alike of species and hybrid production the knowledge must 

 be forthcoming. Thus Lapageria rosea is a parent form which I have chosen for pretty 

 exhaustive description, and though I have tried to select material from what I regard as 

 an average strain, this may still differ from the parent plant used, as several varieties 

 are known to be in cultivation. This may partially explain why it is that hybrids at 



* The contributions that have recently been made (Bot. Central., Bd., xlv. xlvi.) by Schumann are exactly on the 

 lines desiderated, and form a valuable study in tissue variability. 



