IRRITABILITY 251 



transmitted only through hving cells. This is not neces- 

 sarily the case, as MacDougal's experiments tend to prove. * 

 Haberlandtf claims the transmission of the stimulus 

 through tubular series of cells in the phloem of the vascu- 

 lar bundles. Since Fischer ^: has shown the continuity of the 

 sieve-tubes throughout the plant, it is not unlikely that 

 Haberlandt's tubes are also continuous and may therefore 

 furnish at least one course along which the changed turgor 

 of any group of cells could affect other cells. 



The stamens of barberry and of some of the Compositse, 

 and the stigmas of Mimulus, Torenia, etc., contract on 

 being touched. Presumably these, and other similar move- 

 ments, are due to decreased turgor in the parenchyma cells 

 forming a considerable part of the organ, as well as in 



Mimosa,. 



CONCLUSION 



So far we have separately considered the operations of the 

 more evident influences which direct plants in growth and 

 in other activities. This process of analysis leads us to 

 more definite views regarding the effects of these different 

 forces acting as stimuli, but in nature the plant is subjected 

 to all of these forces more or less constantly and more or 

 less simultaneously. Thus light and gravitation may be 

 acting simultaneously and, because of the action of both, 

 the response of an organ to either force will not be the 

 same as if that one were acting alone (see p. 214). Mois- 

 ture, warmth, contact, and chemical substances may also 

 be acting upon the plant at the same time. The behavior 

 of a plant, then, expresses its adjustment to all the influ- 

 ences operating upon it. Its size, form, color, vigor, etc., 

 represent its response tc) all the stimuli it has received. 



Furthermore, as conclusively proved by recent experi- 

 ment, § whatever stimulates or otherwise affects one part or 



* MacDougal, D. T. /. c. fHaberlandt, G. /. c. 



J Fischer, A. Neue Beitrage zur Kenntnlss der Siebrohren. Ber. A. math.- 

 phys. Classe d. K. Sachs. Ges. d. Wiss., 1886. 



§ Hering, F. Cber Wachsthumscorrelationen in Folge mechanischer Hem- 

 mung des Wachsens. Jahrb. i. wiss. Bot., Bd. 29, 1896. Kny, L. Corre- 

 lation in growth of roots and shoots. II. Ann. of Bot., XV., 1901. Other 

 papers cited by these authors, and by Pfeffer, Pflanzenphysiologie Bd. II., 

 Theil 1 1901. 



