26 FOUNDATIONS OF BOTANY 



The seed is gradually tilted until, at the time of their 

 emergence from the ground (at (7), the cotyledons are 

 almost vertical. The only part above the ground-hue (r, Cr, 

 at this period, is the arched hypocotyl. Once out of ground, 

 the cotyledons soon rise, until (at H) they are again ver- 

 tical, but with the other end up from that which stood 

 highest in 0. Then the two cotyledons separate until 

 they once more lie horizontal, pointing away from each 

 other. 



Can you suggest any advantage which the plant derives 

 from having the cotyledons dragged out of the ground 

 rather than having them pushed out, tips first ? 



33. Cause of the Arch. — It is evident that a flexible 

 object like the hypocotyl, when pushed upward through the 

 earth, might easily be bent into an arch or loop. Whether 

 the shape which the hypocotyl assumes is wholly caused 

 by the resistance of the soil can best be ascertained by 

 an experiment. 



EXPERIMENT XII 



Is the Arch of the Hypocotyl due to the Pressure of the Soil on the 

 Rising Cotyledons? — Sprout some squash seeds on wet paper under 

 a bell-glass, and when the root is an inch or more long, hang several 

 of the seedlings, roots down, in little stirrups made of soft twine, 

 attached by beeswax and rosin mixture to the inside of the upper 

 part of a bell-glass. Put the bell-glass on a large plate or a sheet of 

 glass on which lies wet paper to keep the air moist. Note whether 

 the seedlings form hypocotyl arches at all and, if so, whether thfe 

 arch is more or less perfect than that formed by seedlings growing 

 in earth, sand, or sawdust. 



34. What pushes the Cotyledons up? — A very little 

 study of any set of squash seedlings, or even of Fig. 9, is 



