

1901 | GEOTROPISM OF STEMS 413 
the particular tissues concerned in it tothe geotropic irritability. 
With this idea in view, I made a careful study of the histology 
of the root and hypocotyl. Seedlings which have just curved 
are not satisfactory material because of the immaturity of the 
tissues and the amount of plastic material present, and the 
observations on maturer stems which had formerly 
shown themselves positively geotropicmake much , 
study of very young plants superfluous. The @ NS 
apparent line of separation between root and 
hypocotyl is the real one, as to the epidermal 
tissues; the stomatas descend to it, and root 
hairs occur only below it. In the cortical paren- 
chyma there is no sharp distinction between stem 
and root, nor between different parts of the hypo- 
cotyl, not even when, as it matures, chlorophyll 
becomes more abundant in the normally better 
lighted upper part. In the stele® the rotation in 
the arrangement of phloem and xylem is gradual, 
but usually begins below the transition line in the 
epidermis, and is completed at some distance (in 
one very extreme case 17™™) down in the root. ’ 
There is then nothing at all in the histology of £o- Voune 
the hypocotyl that can give a clue to its geotropic Lae of Lupi- 
variability. This does not prove, however, that 2us adbus, showing 
the explanation is to be sought in the finer struct- raaebabe 3 a 
e hypocotyl. 
ure of individual cells. 
The truth is rather that the downward curve occurring in the 
hypocotyl is a response to,a stimulus perceived by the root tip. 
This is not easy of absolute proof, but the evidence seems to 
warrant fully the conclusion. The ‘bent tube”? method of 
demonstration is inapplicable because it is during the very first 
growth that the phenomenon in question occurs. Decapitation 

5 Near the base of the hypocotyl most of the stomata are considerably elevated 
above the general surface. 
SI use the word as a matter of convenience for the bundles and pith. It is not 
Sharply delimited from the cortex. 
