8o 



constriction of the aerial axis, that we can refer in this connection to our 

 subsequent studies in the chapter on "Wounds." 



Figure 2 shows a different root, also from Qiiercus pedunculato, which 

 probably has only been pressed between stones. In meeting with this ob- 

 struction to its growth in length it was bent and. when growing further, be- 

 came flattened. With increasing age the pressed root surface again reached 

 the open and with the removal of the pressure came an increased formation 

 of the wdod ring in great luxuriance like callus rolls. The .squeezing which 

 the roots had undergone, might have acted like girdling and have produced 

 in this a kind of girdling roll above the place of pressure. (See Girdling in 

 the chapter on "Wounds"). 



We can get an idea as to the anatomical conditions in the first stages of 

 such flattening of the root from the investigations of Lopriore^. He 

 observed adventitious roots in the germinating plants of Vicia Paba which 

 were forced to grow under the lateral pressure of cotyledons which had not 

 separated from each other. Within the sphere of pressure these tender roots 

 appeared flattenedlike ribbons but after leaving the region of pressure, they 

 again became normally cylindrical just as was noticd in the oak roots. In 

 the very young roots of the horse bean (Vicia Paba) Lopriore found that the 

 epidermal cells on the sides not pressed upon by the cotyledons had developed 

 into root hairs. On the compressd sides, however, not only the epidermal 

 cells were tangentially flattened but also the two or four outer layers of the 

 bark were considerably pressed so that they formed a kind of peripheral 

 girdle around the root on these sides, whereby the radial walls of these com- 

 pressed cells seem folded zigzag as in a bellows. The cells subjected to the 

 pressure of the cotyledons were also proved changed materially since their 

 membranes either developed into cork or "together with their lumina were 

 impregnated with a kind of protective gum." 



We have already called attention to the fact that in figure i several 

 adventitious roots had been formed above the board-like flattening. As may 

 be seen, the root had made a curve here before entering into the split in the 

 rock and under the influence of this twisting, a new formation of adventitious 

 roots had been started on the free convex side. We perceive in this a result 

 of the stimulus of twisting which XoU- has discussed in detail in his 

 work. It is easy to observe that roots which have l)ecomc twisted because of 

 a pressure, hindering tlieir growth in length, develop new side roots on 

 the convex side at the point of twisting. In water cultures in glass vessels 

 this phenomenon may be observed when strong roots reach the bottom of the 

 vessel and grow against it. 



In mountains emergency precautions are met with in the flatly growing, 

 younger tree roots if the tip of a rootlet has been lost through injury or from 



1 I^opriore, G., Verbanderung infolge des Kopt'ens. Ber. Deutsch. Bot. Ges. 

 Vol. XXII. Part 5, p. 309. 



- Noll, Vergleichende Kulturver.suohe. Sltzungsber. d. Niederrhein. Ges. f. Na- 

 turkunde. Cit. Hot Jahresber. 1900. II. p. 304. 



