INTRODUCTION. 



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 observa- 

 tion, 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 indi- 

 viduals to vary from that average either in under or 

 over development, it may be only of some 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 Hedyclt hint gard- 

 nerianum, when well grown and not overcrowded in a 

 hot-house, sends up flowering shoots which bear on the 

 average 13 lamina-producing leaves, beside one or two 

 basal scales. H. coronarium bears 21, while the hybrid 

 H. sadhrianum bears 17. But not unfrequently from 

 overcrowding, lack of light and nourishment, or other 

 unfavorable surroundings, the number in each may be 

 considerably reduced. Conversely, when very favorable 

 vegetative conditions occur, these are accompanied with 

 greater luxuriance. 



" A shoot of Saxifraga aizoon, with freedom for 

 growth, produces annually 23 to 26 leaves; S. geum, 

 40 to 45 ; and their hybrid, S. andrewsii, 30 to 32. 



" During the autumn of 1890 I happened to go over 

 a large bed of sunflowers, and, in by far the greater num- 

 ber, 27 to 28 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 20 or 21. On the other hand, one in a favor- 

 able situation produced 31 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, etc. 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 speci- 

 men of each for study. 



2. LIMIT OF VARIABILITY. 



" A wide field of patient and laborious work is open 

 in the direction of ascertaining how far the individuals of 

 a species may differ microscopically without losing spe- 

 cific identity. As yet this field may be said to be un- 

 trodden. The contributions that have recently been 

 made (Bot. Central., Bd. xiv, SLVI) by Schumann are 

 exactly on the lines desiderated and form a valuable 

 study in tissue variability, 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 exhaus- 

 tive description, and though I have tried to select mate- 

 rial 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 ex- 

 plain why it is that hybrids at times exhibit a slight 



divergence toward one parent. Again, I shall have to 

 refer at some length to the remarkable change of color 

 exhibited by the flowers of Dmnthus grievei, from white 

 on first opening to rich crimson or crimson-purple on 

 fading. The one parent, D. alpinus, shows scarcely any 

 trace of such floral change, but among the numerous 

 varieties of D. barbatus in cultivation one exhibits the 

 above peculiarity in an equally or even more striking 

 manner. 



" Now, every varietal form inherits certain common 

 specific peculiarities, and also the points that stamp it as 

 a variety, so that one would err in comparing the ordi- 

 nary species with the hybrid. But the very fact that 

 varieties are often inconstant in their varietal details, and 

 do not hand these down in all cases so steadily as a 

 marked species, are reasons for our giving a certain lati- 

 tude in comparison with the hybrid, but equally are 

 reasons for our desiring an exact knowledge of how far 

 a specific form may vary. 



3. COMPARISON OF SIMILAR PARTS. 



" In my earlier investigations it was sometimes 

 found that a certain part or organ of a hybrid did not 

 exhibit intermediate blending of the structure of both 

 parents, but a decided leaning to one. This was at first 

 regarded as an instance of variation from average hybrid- 

 ity, but more careful and exhaustive comparison showed 

 that the apparently exceptional conditions arose from 

 choice of material that did not agree in age, position, or 

 opportunities for growth. Thus I stated in the ' Gar- 

 deners' Chronicle' (April 1890) that while Saxifraga 

 aizoon had many stomata on its upper leaf surface and 

 S. geum had none, S. andrewsii resembled the latter in 

 this respect. Now, I had expected to find some on the 

 leaf chosen from the hybrid, which was one of the lowest 

 of an annual shoot, those of the parents being from the 

 upper parts of shoots. On returning to the matter more 

 recently, it was found that the closely intermediate 

 character of the hybrid was established when leaves of 

 the same relative position and age were chosen. Thus, 

 since 8. aizoon produces on the average 25 leaves annually, 

 the hybrid 32, and 8. geum 40, if the tenth leaf from the 

 base be chosen in the first, we should select the four- 

 teenth in the hybrid and the eighteenth in the other 

 parent. The same principle of judicious selection of 

 material must be applied not only in dealing with large 

 organs but also in minuter details, such as bundle ele- 

 ments, matrix cells, and sclerenchyma, as well as starch 

 grains, chloroplasts, and other cell products. 



4. AVAILABLE LIMIT FOR COMPARISON OF PARENTS WITH THEIB 

 HYBRID PROGENY. 



" During the last decade problems bearing on the 

 relative potency of the male and female elements in the 

 development of an organism have been greatly discussed. 

 The present investigation not only throws great light on 

 these, but will enable us to compare more accurately than 

 hitherto the capabilities of each sex element. It is mani- 

 fest, however, that when a hybrid is the product of 

 parents that are widely divergent in histological details 

 the comparison will be easy, but when we attempt to 

 compare a hybrid with two parents which are regarded 

 as species, but whose chief specific differences are those 

 of coloring and size, it is almost or quite impossible to 



