358 



SCIENCE 



[N. S. Vol. XXVIII. No. 716 



etative growth for six years; while by 

 removing a fragment of the plant and 

 cultivating it in other conditions the re- 

 productive organs could at any time be 

 made to appear.^^ 



Chlamydomonas media, a unicellular 

 green alga, when grown in a 0.4 per cent, 

 nutrient solution, continues to increase by 

 simple division, but conjugating gametes 

 are formed in a few days if the plant is 

 placed in pure water and kept in bright 

 light.^* Numberless other cases could be 

 given of the regulation of form in the 

 lower organisms. Thus Sporodinia grown 

 on peptone-gelatine produces sporangifer- 

 ous hypha, but on sugar zygotes are 

 formed. Again, Protosiphon botryoides, 

 if grown on damp clay, can most readily 

 be made to produce spores by transference 

 to water either in light or in darkness. 

 But for the same plant cultivated in 

 Knop's solution the end can best be ob- 

 tained by placing the culture in the dark.^^ 

 Still these instances of the regulation of 

 reproduction are not so interesting from 

 our point of view as some of Klebs's later 

 results.^" Thus he has shown that the 

 color of the flower of Campanula trache- 

 lium can be changed from blue to white 

 and back again to blue by varying the con- 

 ditions under which the plant is culti- 

 vated. Again, with Sempervivum" he 

 has been able to produce striking results 

 — e. g., the formation of apetalous flowers 

 with one instead of two rows of stamens. 

 Diminution in the number of stamens is a 

 common occurrence in his experimental 

 plants, and absolute loss of these organs 

 also occurs. Many other abnormalities 

 were induced, both in the stamens and in 

 other parts of the flowers. 



'■^ Willkiirliche Entwick., p. 27. 



" Klebs, Bedingungen, 1896, p. 430. 



'=BioL CentralU., 1904, pp. 451-3. 



^'Jahrb. f. wiss. Bot., XLII., 1906, p. 162. 



" Abhandl. Naturforsch. Ges. zu Halle, XXV., 

 1906, pp. 31, 34, etc. 



There is nothing new in the character 

 of these facts ;^* what has been brought to 

 light (principally by the work of Klebs) 

 is the degree to which ontogeny is con- 

 trollable. We are so much in the habit of 

 thinking of the stable element in ontogeny 

 that the work of Klebs strikes us with 

 something of a shock. Most people would 

 allow that change of form is ultimately 

 referable to changed conditions, but many 

 of us were not prepared to learn the great 

 importance of external stimuli in onto- 

 geny. 



Klebs begins by assuming that every spe- 

 cies has a definite specific structure, which 

 he compares to chemical character. Just 

 as a substance such as sulphur may assume 

 diiferent forms under different treatment, 

 so he assumes that the specific structure of 

 a plant has certain potentialities which may 

 be brought to light by appropriate stimuli. 

 He divides the agencies affecting the struc- 

 ture into external and internal conditions, 

 the external being supposed to act by caus- 

 ing alterations in the internal conditions. 



It will be seen that the scheme is broadly 

 the same as that of Pfeffer for the case of 

 the movement and other temporary reac- 

 tions. The internal conditions of Klebs 

 correspond also to the "physiological state" 

 of Jennings. 



From what has gone before, it will be 

 seen that the current conception of stim- 

 ulus^' is practically identical, whether we 



" See the great collection of facts illustrating 

 the " direct and definite action of the external 

 conditions of life " in " Variation of Animals and 

 Plants," II., p. 271. 



" With regard to the terminology of stimula- 

 tion, I believe that it would greatly simplify mat- 

 ters if our classification of causal conditions 

 could be based on the relation of the nucleus to 

 the rest of the cell. But our knowledge does not 

 at present allow of more than a tentative state- 

 ment of such a scheme. It is now widely believed 

 that the nucleus is the bearer of the qualities 

 transmitted from generation to generation, and 

 the regulator of ontogeny. May we not, therefore. 



