164 THE CAUSE OF SPECIFIC SHAPE 



Correlative influences play a very important part in the progress of 

 development, and the interactions between the adult and growing parts 

 are such as to result in the maintenance of a certain condition of labile 

 equilibrium. The modification or commencement of any form of activity 

 in one organ may influence other organs as well. This is especially well 

 shown by the influence of the developing embryo upon the ripening carpel 

 and upon the fruit-stalk 1 . 



Flowering and fruiting involve a certain reduction of vegetative 

 activity, which is therefore favoured by removing the flowers 2 . Conversely, 

 great vegetative activity tends to decrease or suppress the formation of 

 reproductive organs in both higher and lower plants 3 . Hence in the 

 case of the potato a decreased 'formation of tubers is accompanied by an 

 increased formation of flowers 4 . It is owing to a reaction directed towards 

 the replacement of missing parts and the performance of a particular 

 function that in Onoclea struthiopteris the removal of the foliage leaves 

 results in sterile leaves developing in the place of fertile sporophylls 5 . 

 Similarly such plants as wallflowers can be kept in flower nearly the whole 

 of the year by preventing any formation of fruits. 



In the normal progress of development meristematic cells and 

 primordia frequently, have their growth temporarily or permanently 

 inhibited, and it is in correlation with the direction of growth to a 

 particular purpose that the sporangiophore of Phycomyces almost entirely 

 ceases to elongate during the formation of the sporangium 6 . Correlative 

 repression in many cases goes so far as to cause the death of particular 

 cells and primordia, as for example takes place during the formation of 

 tracheae, and of certain flowers. 



SECTION 46. Correlation. 



The nature of the reaction affords no criterion as to whether it is 

 directly or indirectly produced in the reacting organ, for a mere change 

 in the tone or activity in the cells abutting upon the reacting organ may 

 produce a pronounced formative effect. A stimulus acting on one cell 

 may affect others, and hence it is impossible always to say whether the 



1 Hofmeister, Allgem. Morph., 1868, p. 634 ; Reinke, Nachr. d. Ges. d. Wiss. zu Gottingen, 

 1878, p. 473. 



2 Mattirolo, Sulla influenza che la estirpazione dei fieri esercita sui bitorzoli radical!, 1900, 

 p. 38. 



3 Mobius, Beitr. z. Lehre v. d. Fortpflanzung, 1897, p. 132; Goebel, Organography, 1900, 

 I, p. 182 ; Miiller-Thurgau, Landw. Jahrb., 1898, Bd. xii, p. 57. As regards algae cf. Klebs, Jahrb. 

 f. wiss. Bot., 1900, Bd. xxxv, p. 150. 



4 Knight, Phil. Trans., 1806, Part n, p. 298 ; C. Kraus, Forsch. a. d. Gebiete d. Agriculturphysik, 

 1881, Bd. iv, p. 58 ; Vochting, Bibl. bot., 1887, Heft 4, p. 20. 



5 Goebel, Ber. d. Bot. Ges., 1887, p. 69. 



6 The growth of Pilobolus ceases with the formation of sporangia, and hence continues in 

 darkness, which prevents the production of sporangia. 



