PHYSIOLOGY 



28; 



far observed, took place ill the growing portions of plants, and 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 still able, although 

 .slowly, to exhibit geotropic curvatures ( 8S ). THE NODES OF GRASSES SHOW THAT 



BESTING TISSUES ALSO CAN HE 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 barrel-shaped thickenings of the leaf-sheaths above 

 their actual insertion 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 

 unt'requently 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 



Km. _>:>. (ieotropic erection n( a grass-haulm 

 liy tin- curvature of a node. 1, Placed 

 horizontally, lx.it h sides (. ./) of the node 

 being of equal length ; '2, the under side 

 () lengthened, the upper side (o) somewhat 

 shortened ; as a result of the curvature 

 the grass-haulm has been raised thrmigli 

 an angle of 75. 



Fli:. _':!!.- Seedling of ' '< rhi/n. The peg 

 (W) (levelo)ied from the under side of the 

 hyjiocotyl assists in rupturing the seed- 

 coat. 



pressure, and lo>s (if water, knee-like curvatures are formed at the nodes, by means 

 of which the haulm is again quickly brought into an erect position (Fig. 230). 



Seedlings of Cucurbitaceae. The stimulus of gravity induces the growth of a 

 peculiar peg-like outgrowth from the lower side of the hypocotyl of the seedling of 

 <.'iu-in-lnta (Fig. 2:31 W . This peg, which results from a limited geotropic growth 

 in thickness, assists in liberating the cotyledons from the seed-coat. 



Modifications in the character of the geotropism, as of the helio- 

 tropism, of an organ may be occasioned by the operation of internal 

 as well as external influences. Such changes in their geotropic 

 position frequently occur, as VOCHTING has demonstrated, during the 

 development of flower-buds, flowers, and fruits, whose movements 

 are, according to WIESNER, especially influenced by epinastic load- 

 curvatures (p. 272). The burial of the fruits of Trifolium suUerr,i<in,< 

 and Anichis Itj/iiogaea is due to geotropism. Of the changes in the 

 geotropic conditions of plant organs due to external causes, those are 

 particularly noticeable which result from a failure of a sufficient 

 supply of oxygen, by which, for example, roots and rhizomes are 

 made negatively geotropic. The temperature influences the position 



