62 



L. LENORE CONOVER 



months or a year into a twining climber that may rise to a height 

 of 10 meters. 



In the study which follows special attention has been given to 

 these features: (1) The directive agents for the horizontal growth 

 of the apex of the shoot ; (2) The presence of morpho-physiologi- 

 cal upper and lower sides; (3) Nutation of tip of main axis and 

 of lateral branches; (4) Phenomena of climbing. 



I. DIRECTIVE AGENTS FOR THE HORIZONTAL GROWTH OF THE 



SHOOT 



In the matter of changing its direction after a certain stage 

 of development is reached, the stem of Cucurbita pepo resem- 

 bles that of Asparagus plumosus. The hypocotyl of Cucur- 

 bita pepo is always orthotropic, but the epicotyl curves just 

 above the cotyledons and becomes plagiotropic. Sachs^ sup- 

 posed cases like this to have their plane of curvature deter- 

 mined by the direction of light, but believed the assumption 

 of the plagiotropic position to be due in some cases to gravi- 

 tation, in some to light, in some to epinasty, and in some to a 

 combination of stimuli. Czapek,^ working on the cause of 

 bending of the hypocotyl of Cucurbita, came to the same con- 

 clusion as Sachs. 



The orthotropic shoot of Asparagus is very sensitive to the 

 directive influence of light as can be shown by one-sided illu- 

 mination. At a temperature of 20°C. the tips of several young 

 shoots were brought from the vertical to the horizontal position 

 by the light stimulus within an hour. In another preparation, 

 a pot with a young orthotropic shoot 15 cm., high was laid in 

 a dark-box with a small window for one-sided access of light. 

 The shoot extended horizontally and was transverse to the 

 direction of light. The test lasted for an hour during which 

 the pot was twice shifted so as to keep the moving tip more 

 nearly transverse to the incident light. In this period the tip 



^ Sachs, Ueber orthotrope und plagiotrope Pflanzentheile. Arbeit. Bot. Inst. 

 Wurz. 2: 226. 1882. 



' Czapek, Studien iiber die Wirkung ausserer Reizkrafte auf die Fflanzengestalt. 

 Flora 85: 424. 1S98. 



