The Life of the Plant. 201 
wall, or even in a hard siliceous coating, as in the case 
of the Diatomacez ; but it is believed that the cause of. 
their active creeping motion must be that the protoplasm 
projects through their shell at certain spots, and can be put 
out and withdrawn like a foot. The Oscillatoriez consist of 
long filaments which often oscillate rapidly ; and the Spiru- 
lineze are filaments of similar form which coil about in a 
great variety of ways. The cause of their movements has 
not been ascertained. 
Vutatzon is the term given to those curvatures which cause 
parts of plants that are growing in length to assume suc- 
cessively different directions without any apparent external 
cause. Examples are furnished by the flowering scapes of the — 
onion, AMium Cepa, which, before they attain their ultimate 
length, bend slowly first to one side and then to another ; or 
still better, by climbing stems. In these, such as the hop and 
the scarlet-runner, the first internodes do not twine ; but when 
the apex of the stem hangs down in consequence of its 
weight, it begins to revolve in a circle, and thus to approach. 
the support which it afterwards embraces. The earlier coils 
are hence looser than the later ones, which embrace the sup- 
port more closely. These curvatures of nutation appear, in 
general, to take place at the spots where the tension of the 
tissues 1s the greatest, and to be caused by the growth in 
length being greatest first on one and then on the other side. 
Parts of plants which are growing in length sometimes 
become, by long-continued close contact with a solid body, 
firmly attached or adherent to it. . Instances are afforded in 
the adhesion of pollen-tubes to the stigmatic hairs and to 
the internal wall of the ovary ; and in the climbing and turn- 
ing of the tendrils of many plants round slender supports. 
Tissues in a state of tension become relaxed by con- 
cussion. Parts of a great number of plants, when in a state 
of active growth, undergo a diminution of the elasticity of 
their tense tissues by violent shaking, repeated bending, a 
blow on one side, or sometimes even simply by stretching, 
