50 PENHALLOW ON MECHANISM 
referred primarily and in general terms to conditions of growth, of which they are the 
result. They may arise, however, as already pointed out, either from unequal growth 
and nutrition of parts, or from special conditions of turgescence, one or both combined. 
Or, as Sachs * states, “in those movements which occur during growth, the tension of the 
tissues is concerned only so far as any change in it reacts on growth and modifies it. 
Periodic movements, and those due to irritation, on the contrary, depend entirely on 
changes in the the tension of the tissues, which, in this case, are fully developed only 
when the organ has attained maturity.” 
These general principles apply to all the subjects now under consideration, and 
accepting them as tenable, we shall not in the present paper concern ourselves more 
particularly as to the special physiological changes involved, and whatever references are 
made to growth are to be accepted in the general meaning of that term, unless otherwise 
specified. Two general considerations are of importance in this connection, viz., the 
mechanical value of the tissues, and the continuity of protoplasm. 
Of the various tissues which enter into the composition of motile organs, parenchyma, 
collenchyma, bast and wood, are of chief value. Of these, the parenchyma probably 
stands first as capable of the most rapid growth and the most extreme variations of 
tension from turgescence or other cause. The collenchyma undoubtedly stands next 
in both of these respects ; while the bast, from its more permanent character, as well as 
from the results obtained by both Schwendener* and Haberlandt,* in which the great 
elasticity of this tissue appears, is in all probability the most important mechanical 
element, by reason of the retarding influence it exerts upon the growth of the more 
rapidly extending and external parts. 
The inference which naturally follows from this is, that the principal conditions of 
tension with reference to elongation, are established and maintained primarily between 
the parenchyma and collenchyma on the one hand, and the bast and other vascular 
elements on the other ; and secondarily, between the parenchyma and the collenchyma. 
It will also follow that, whenever one of these last-named tissues is in excess, it must 
exert a preponderating influence in changes of tension, without special reference to its 
particular capacity for such variations. 
One of the most important factors in the physiology of motion, particularly that due 
to irritation, is the continuity of protoplasm. This fact has now been observed in so many 
widely different cases, and involves so little difficulty in its determination in almost any 
living tissue, that we can no longer regard its application as a general law, with reason- 
able doubt ®. This law is of so recent origin, however, that at present but little is known 
as to its precise relation to motion; but that it is connected with it in those cases where 
there is distinct transmission of impulse to parts somewhat remote from the centre of 

? Darwin, Movements of Plants, p. 2. Sachs, Vorlesungen über Pflanzen-Phys. p. 775. 
? Text-book, 2nd Ed. p. 878, ete. Morren, La Sensibilité et la Motilité des Veg. Bruxelles, 1885, p. 52, etc. 
* Das Mechanische Princip. im Anatomischen Bau der Monocotylen. Leipsig, 1874. 
* Physiologische Pflanzen Anatomie. 
5 Bot. Centralbl. xiv. 89—121. Proc. Royal Soc., xxxv. 163. Zbid, xxxiv. 272. Jahrb. Wiss. Bot., xii. 170. 
Vorlesungen über Pflanzen-Physiologie, Sachs., 102. Nature, xxx. 182. xxxi. 337, 290, 390. Quart. Jour. Mic. 
Sci., Oct. 1882. Phil. Trans. Royal Soc., 1883, 817. Flora, 1863, 68. Hanstein, Die Milchsaftgefässe, 1864. Wil- 
helm, Zur Kenntniss des Siebrohrengefiisse Dicotyler. Pflanzen, 1880. 
