THE PilEXOMENA OF GEKMINATION. 
93 
Another cause of incitement in the protoplasm of the root-tip is the 
presence of moisture. An experiment of Sachs illustrated this remark- 
ably well. He gre^v some Beans in wet moss in a trough made of 
wdre gauze. The radicles grew vertically downwards, as long as they 
Avere inside and equally moistured all round, but on escaping through 
the meshes the attraction of the moist moss was greater than that of 
gravity ; so that the tip turned back and re-entered the trough, but by 
being now in the same condition as at first the radicle soon curved down- 
wards again. On repeating the process it actually threaded its way in 
and out of the meshes of the gauze. The trough was suspended at a 
considerable angle so as to render the attraction of the water more 
evident. 
The next movement to bo noticed is the circumnutation of the radicle 
when elongating downwards. It consists of an approximately circular 
bowing motion, but may more correctly be described as an irregular 
waggling. This presumably enables the radicle to find the line of least 
resistance to its penetrating the soil. The root-hairs formed by the 
elongation of epidermal cells help to fix it by adhering to the particles of 
ihe soil, thus giving a greater purchase for penetration. 
The strength of the growing root-tip was ascertained approximately 
by Darwin in the case of a Bean, to be sufficient to raise one-quarter of 
a pound in twenty-four hours. The root of another Bean was passed 
through a round hole in a piece of wood cleft to the end. The strength 
of the radicle exerted in opening the cleft was 8 lb. 8 oz. Only -1 inch 
was within the hole, and the root had really exerted a greater transverse 
strain, for it had split the solid wood for a length of rather more than 
a quarter of an inch. 
Another discovery made by Darwin was that the root-tip is sensitive 
to pressure, and that if it be continuous the root turns away from the 
obstruction. He grew seeds in moist air and fixed a piece of card at 
one side of a root-tip. This then curled up on the opposite side, often 
forming a complete circle in so doing. Darwin observes that this is the 
only known instance of a vegetative organ avoiding and not turning 
round an object, as in climbing plants. 
As soon as the radicle is well out of the seed-skin, the plumule may 
grow or be temporarily arrested. In the latter case the radicle comes 
above ground and forms the aerial stem and bears the two, now green 
cotyledons, as in Mustard. 
If, however, the cotyledons remain below ground then the plumule rises 
at once, as in the Oak. The stem circumnutates as well as the radicle, and 
this assists it in freeing itself from the superincumbent soil ; and if it be 
a stem-climber it utilises the power of circumnutation for twining up 
other plants. 
The interpretation of the erect growth of stems is that it is due to 
light acting as an irritant, for plants are illuminated from above. If 
plants be grown from seed on a gauze shelf in an inverted box and 
illuminated only from below, then the shoots grow vertically downwards 
in the direction both of the light and gravity, showing that the first is 
the real cause, and not gravity, of erect growth. 
The only effect of gravity upon a stem growing upwards is to make 
