MOVEMENTS OF THE SEEDLING 23 
29. What pushes the Cotyledons up?— A very little 
study of any set of squash seedlings is sufficient to show 
that the portion of the plant where roots and hypocotyl 
are joined neither rises nor sinks, but that the plant grows 
both ways from this part. It is evident that as soon as 
the hypocotyl begins to lengthen much it must do one of 
two things: either push the cotyledons 
out into the air or else force the. root 
down into the ground as one might push 
a stake down. What co, 
A 
A B c D E 
Fic. 9. Successive Stages in the Life History of the Bean Seedling. 
AA, the surface of the ground; 7, primary root; 7”, secondary root; 
c, hypocotyl; a, arch of hypocotyl; co, cotyledons. 
changes does the plantlet undergo in passing from the 
stage shown at A to that of B and of C, making it harder 
and harder for the root to be thrust downward ? 
30. Use of the Peg. — Squash seedlings usually (though 
not always) form a sort of knob on the hypocotyl. This 
