262 BOTANY 



When the apex of a sinistrorse shoot points towards the north, it is the east 

 side of which the growth is promoted by geotropism ; in dextrorse climbers, on the 

 contrary, the growth of the west side is more rapid. That the same stimulus affects 

 in different plants the growth of opposite sides, may be explained by the difference 

 in the arrangement of their irritable structures (through their reversed position) 

 within the organ. From the fact that the promotion of growth occurs always on 

 the same side, it will be apparent that the apex of an inverted twining stem must 

 unwind from its support. (Concerning the behaviour of stem-climbers on the 

 Klinostat, compare p. 264.) 



Curvature of Grass -Haulms. — All the examples of geotropic 

 movements, so far observed, took place in the growing portions of 

 plants, and whether occurring in unicellular or multicellular organs, 

 were due to a disturbance of the course of growth. A curvature even 

 of lignified twigs can also be produced by the one - sided stronger 

 growth of the cambium and of the young secondary tissues. Even 

 many-year-old branches of Conifers are all able, although slowly, to 

 exhibit geotropic curvatures. The nodes of grasses show that 



RESTING "TISSUES ALSO CAN BE EXCITED TO NEW GROWTH BY THE 



stimulus of gravitation. The knot-like swellings on the haulms 

 of the Grasses are not nodes in a morphological sense, but are 

 cushion-like thickenings of the leaf-sheaths above their actual in- 

 sertion on the shoot axis. The part of the stem thus enveloped 

 is very tender and flexible. When a grass-haulm is laid horizontally, 

 which not unfrequently occurs through the action of the wind or 

 rain, the nodes will begin to exhibit an energetic growth on their 

 lower sides. As the upper sides of the nodes take no part in the 

 growth, but are instead frequently shortened through pressure and 

 loss of water, knee - like curvatures are 

 formed at the nodes, by means of which 

 the haulm is again quickly brought to an 

 erect position (Fig. 207). In this way laid 

 corn is able to right itself. Similar curva- 

 tures to those of the grass nodes may 

 be produced in the true nodes of the grass- 

 like Dianihi, and of the Polygonaceae (Poly- 

 Fig. 207.— Geotropic erection of a gonum, Rwmex) and Commelinaceae (Tradcs- 



grass-haulin by the curvature of Q^fififA 

 a node. 1, Placed horizontally. 



Modifications in the character of the geo- 

 being of .equal length ; 2, the tropism, as of the heliotropism, of an organ 



under side («) lengthened the bg occasioned by t h e operation of in- 



upper side (o) somewhat short- J t - n n 



ened;asaresuitofthecurvature ternal as well as external influences. Such 

 the grass-hauim has been raised changes in their geotropic position frequently 

 occur, as Vochting has demonstrated, dur- 

 ing the development of flower-buds, flowers, and fruits (buds and flowers 

 of Papaver, flowers and fruits of Aquilegia, Delphinium, Aconitum, and 

 in the burial of the fruit of Trifolhm mbtnraneum, Arachis hypogaea, etc.). 



both sides (u, o) of the node 



through an angle of 7 



