688 



LECTURE XXXIX. 



ever; tlie curvature of the node is due to the fact that its under side when placed in 



the unwonted horizontal position becomes 

 elongated vigorously by means of growth, 

 while the upper side grows feebly or 

 not at all, and in fact often shortens 

 considerably. If the piece of haulm with 



Fig. 387.— a piece of the haulm of the wheat ^ ' 



[^ards""' '"''' '"'"^°"'^">'' "■'"' ''^ """^^^ '^""■'''^ the curved node is turned so that the 



convex side comes to lie uppermost, then 



Mi^ , / W 1] / jr ^^^^^ was previously the upper side of the 



/|i|uP\ iV / V \AvV^ ^ node which had become concave begins 



y\ 3t;.^=J::r:^-_ '^^ //^;;?^T / 1 to grow also, and just as strongly ; it 



therefore becomes straight and is thus at 

 last much elongated, whereas a similar 

 node on a piece of haulm treated in the 

 same way, but which remains erect, shows 

 no elongation of any kind. The convex 

 under side of a strongly curved node ap- 

 pears smooth, translucent, and bright ; the 

 concave upper side dark and rough from 

 minute transverse folds. On this side more- 

 over there is often perceived a deep in- 

 folding, looking as if the node had been 

 artificially bent till it cracked; this evi- 

 dently depends upon the fact that the 

 lower side as it vigorously elongates and 

 curves upwards compresses the tissue of 

 the passive upper side, a process which is 

 intensified by the latter in its turn losing 

 water, as can be concluded with certainty 

 from the very considerable shortening. Of 

 my very numerous measurements in this 

 connection, I will only quote one in illus- 

 tration of what has been said. A node of 

 the Maize, about 12 mm. thick, was 5 mm. 

 long on all sides : after it had lain six days 

 in a horizontal position the upper side had 

 become shortened by 0-5 mm. (i. e. y^^th of 

 its original length), while the lower side 

 had become elongated by 7*5 mm., i.e. 

 to 1 2-5 mm. In other cases, however, the 

 shortening of the upper side was still 

 more considerable, (e. g. from 4 mm. to 

 3,) while the length of the under side 

 increased from 5 mm. to 11. Microscopic 

 examination shows that the cells of the 

 epidermis and of the i)arenchyma of the lower half of the node have elongated 



Fig. 388. —Portion of a Scarlet Runner 

 originally growing erect, has been for some hours turned 

 upside down to show the geotropic curvatures of the 

 motile organs P, />,, Pi. 



