IN IK K.N 



It was this, well illustrated in the group *'>rripedia, 

 which f>-r.. 1 Kuwin -!..ly )>ut surely to frame and 

 iiiinciatf hi* c\<>luti"ii liv|H>the*ia. 



\- jiLint afti-r plant has pa.--<-l under my ok- 

 tioii. 1 h. .:ly impressed, not only with Uie 



averauf Mimlarity in devrlnpinerit that each shows, but 



m..n- with the constant tendency there is for null- 

 >; liial* to vary from that average either in un.l. r r 

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



some large organ. As illustrations on a somewhat 

 Urge scale, 1 may refer to the number, position on the 



and SIM of leaves, a line of inquiry which has been 

 entirely overlooked by systentatists, but which can afford 

 ehara :. i. iM.' \.ilui-. 'Vl\u* Jlnlyrhium gard- 



:num. win n w.ll jrrowu and not overcrowded in a 

 hot-house, >m.U up Dowering shoots which bear on tin- 

 average 13 lamina-pr.'-liMiig leavea, betide one or two 

 basal scales. // cunmarium bean 21, while the hybrid 

 //. ladlerianum bears 17. But not unfrequently from 

 rowding, lack of light and nourishment, or other 

 unfavorable surroundings, the number in each may be 



K-rably reduced. Conversely, when very favorable 

 vegetative conditions occur, these are accompanied with 

 grctt-r luxuriance. 



\ shoot of Sari/ruga aizoon, with freedom for 

 f !i, produces annually 23 to 26 leaves; 8. gtum, 



in.) tlu-ir l.yl.n.l. S. andretrsii, 30 to 32. 

 " I luring the autumn of 1890 I happened to go over 

 a lar^-r !! f sunflowers, and, in by far the greater num- 

 ber. 2? to 28 leaves were formed between the cotyledon* 

 and terminal capitulum. A few instructive caws of 

 variability from the avenge were noted. The bed was 

 one which sloped to the son and some plants at the back 

 that were slightly overshadowed by trees had been starved 

 in t!ii-ir light and moisture supply. Their leaves were 

 20 or 21. On the other hand, one in a favor- 

 able situation produced 31 leaves. 



' Hut 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, et<-. In the choice of material, therefore, for 

 hyl.rul investigation one should either be acquainted 

 with tin* 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 VABIABIUTT. 



" 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- 



. 1. i.nty. As yet this field may be said to be un- 

 f"-i contributions that have recently been 



made (Bot Central., ltd. xiv, XLVI) 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 



!::< must be forthcoming. Thus Lapageria rosea 

 is a parent form which I have chosen for pretty exhaus- 



.-soription, and though I have tried to select mate- 

 rial from what I regard as an average strain, this may 

 itill differ from the parent plant used, as seven! 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 

 rxlnl, :{.! by the flowers of I'uinthu* grievri.lTom white 

 n tirat opening to rich crimson or crimson-purple on 

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

 trace of such floral change, but among the numerous 

 \ari.-tu-s of It. barbatut in cultivation one exhibits the 

 above peculiarity in an equally or even more striking 

 manner. 



" Now, every varietal form inherits certain 

 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. 



J. COMPABISON OF SlMILAB PABTS. 



" 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- 

 denen' Chronicle' (April 1890) that while Sarifnga 

 aizoon had many stomata on its upper leaf surface and 

 S. geum had none, 8. andrewni 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 S. 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 FOB COMPABIBON or PABEXTS WITH THUS 

 HTBBID PBOOENT. 



"During the last decade problems bearing on the 

 relative potency of the male and female elements in UM 

 development of an organism have been greatly ditcuased. 

 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, bat when we attempt to 

 compare a hybrid with two parent* which are regarded 

 as species, but whose chief specific differences are those 

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



