DEVELOPMENT AND REPRODUCTION 297 



It appears that there are substances in the flowers of some plants that pre- 

 vent fertilization by pollen from the same individual, thus resulting in self -ster- 

 ility. If the pollen is appHed to the stigmas of another individual of the same 

 species, however, fertilization is not thus prevented.' In such cases the poUen, 

 to be effective, must be applied to an individual of different sexual origin from 

 the one that bore it. 



Parthenogenesis is also controlled by external conditions. Nathansohn,^ for 

 instance, succeeded in obtaining parthenogenetic reproduction in several species 

 of the genus MarsUia by subjecting the spores to high temperatures. 



Higher plants propagate themselves vegetatively,by means of tubers, bulbs, 

 etc. An organ, or even a. portion of an organ, removed from the plant, may 

 generate a new individual.' For instance, if a Begonia leaf is cut off and laid 

 upon moist sand, adventitious roots are formed and a new leafy branch develops. 

 If the leaf is taken from a plant that is in bloom the branch that develops bears 

 flowers instead of being leafy. Fig. 172 shows a leaf of Achimenes haageana 

 that was taken from a plant just about. to bloom; flowers have been developed 

 instead of leaves.* Leafy branches or flowers may be obtained at will, by cutting 

 the leaves for propagation from plants in the proper stage of development. It 

 thus appears that the leaves of a plant about to bloom contain different chemical 

 substances from those found in the leaves of earlier developmental stages.* 



The ancient Greeks were already aware that if a bud is taken from one plant 

 and grafted upon another a new branch is produced by the development of the 

 bud, and that this branch retains the special character of the plant from which 

 the bud originally came. The operation of grafting, known to gardeners for so 

 long a time, fiumshes the physiologist with a valuable means for studying the 

 processes of growth and metabolism. Vochting' has collected the scattered 

 literature of this subject and has employed the surgical term transplantation to 

 designate all kinds of coalescences between plant parts. 



Experiments have shown that widely different portions of plants may be 

 brought together and made to coalesce. Even the transplantation of a leaf 



' Correns, C, Sdbststerilitat und Individualstoffe. Festschr. (84 Versamml.) Deutsch. Naturf. u. 

 Aerzte, med.-naturwiss. Ges. P. 186-217. Munster i. Westf., 1912. 



' Natfaansolm, Alexander, Ueber Parthenegenesis bei Marsilia und ihre Abhangigkeit von der Tem- 

 peratur. Ber. Deutsch, Bot. Ges. 18: 99-109. 1900. 



• Goebel, K., Ueber Regeneration im Pflanzenreich. Biol. Centralbl. 22: 385-397, 417-438, 481-sos. 

 1902. [In this connection see also; Loeb, Jacques, Rules and mechanism of inhibition and correlation in 

 the regeneration of Bryophyllum calycinum. Bot gaz. 60 : 249-276. 1915. Idem, Further experiments on 

 correlation and growth in Bryophyllum calycinum. Ibid. 62 : 293—302. 1916. Idem, On the association 

 and possible identity of root-forming and geotropic substances or hormones in Bryophyllum calycintim. 

 Science, n. a. 44; 210-211. 1916. Idem, Influence of the leaf upon root formation and geotropic curvature 

 in the stem of Bryophyllum calycinum. and the possibility of a hormone theory of these processes. Bot. 

 gaz. 63 : 25-50. 1917. Idem,, A quantitative method of ascertaining the mechanism of growth and of in- 

 hibition of growth in dormant buds. Science, n. s. 45 : 436-439. 1917. Idem, The chemical basis of re- 

 generation and geotropism. Ibid. 46: 11S-118. 1917. Idem, The organism as a whole. X+ IS3 p. 

 New York, 1916.] 



* Goebely Karl E., Organographie der Pflanzen, inbesondere der Archegoniaten und Samenpflanzen. 

 Jena, 1898— 1901. Part I, p. 41. [Idem, Organography of plants especially of Archegoniatee and Sper- 

 maphyta. Translated by Isaac Bayley Balfour. 2v. Oxford, 1900-1905.] 



* EZlebs, Georg, Ueber die Nachkommen kiinsthch veranderter Bluthen von Sempervivum. Sitzungsber. 

 (math.-naturw. Kl.) Heidelberg. Akad. Wiss. 19096:1-32. 1909. 



• Vochting, Hermann, Ueber Transplantation am Pflanaenkorper. Tubingen, 1892. 



