GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 779 
A second general property of the organised parts of plants is that they change 
/heir form when the external conditions remain perfectly unaltered, internal changes 
being the only efficient cause. Almost every process of growth is associated with 
change of form. These facts may be more briefly described by ascribing to 
organised structures endowed with the power of growth internal forces or plastic 
tendencies, if it is clearly understood that the term is only used to express a still 
unresolved aggregate of causes. As a result of these internal forces, organised 
structures have the power of overcoming resistance. Thus, for example, plasmodia 
which are constantly altering their form are able, notwithstanding their gelatinous 
and very soft nature, to overcome their own weight, and to creep up solid bodies. 
In the same manner the growth of the wood takes place with such force as to over- 
come the very considerable pressure of the surrounding bark. 
But although the internal causes of these plastic tendencies are able to over- 
come certain obstacles, it is on the other hand certain that growth is also influenced 
by external forces, such as pressure, traction, stretching, bending, &c., which are 
able to alter the form of solid bodies. The observations which have been made on 
this subject will be collected in the following sections ; but it is in the first place 
necessary to define certain terms which will frequently be employed. 
Like unorganised solid bodies, those which are organised oppose a greater 
or less resistance to the external forces which tend to alter their form ; and are 
hence divided into hard and soft bodies. A hard body is one which offers con- 
siderable resistance, like many lignified or silicified cell-walls ; a soft body is one 
which offers very litde resistance, like protoplasm, chlorophyll-granules, or swollen 
cell-walls which have ceased growing, as gum-tragacanth. Structures which become 
disintegrated under pressure and traction rather than undergo any considerable 
change of form are brittle, like grains of starch or crystalloids of aleurone. If, on 
the contrary, they are capable of undergoing considerable changes of form, whether 
this take place by pressure or traction, they are extensible. It is clear that flexibility 
depends to a certain extent on extensibility, since the side of the bent part which 
becomes concave is compressed, the convex side stretched. All these properties are 
relative, and the same body may exhibit different phenomena according to the 
nature of the external forces which act upon it. Thus, for example, under a 
sudden blow the apex of a root behaves like a brittle body, and breaks easily, while 
it is flexible if slowly bent. 
If the form of an extensible body has been changed by pressure, traction, or 
bending, and if, when then left to itself, it retains the form to which it has been 
forced, it is called inelastic ; if, on the other hand, it resumes its original form, it 
is elastic. If the changes of form produced by external causes are small, they are 
usually completely reversed when the body is left to itself, and within these limits 
the body is perfectly elastic ; but if the change of form exceeds certain limits 
dependent on the nature of the body and the length of time during which the force 
has been acting, it does not again assume exactly its previous form. The greatest 
amount of change which yet permits a complete restoration of the original form 
determines the Limit of Elasticity of the body; when this is exceeded, the stretched 
substance partially retains the form which it has been made to assume, and the less 
complete the return to its primitive shape the more imperfect is its elasticity. It 
