7 26 FOUNDATIONS OF BOTANY 
The seed is gradually tilted until, at the time of their 
emergence from the ground (at C), the cotyledons are 
almost vertical. The only part above the ground-line G, G, 
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 C. 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 the 
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 
