458 PHYSIOLOGY 
Growth movements and turgor movements. The intimate relations that exist 
between turgor and growth, as well as the suddenness of their response under favor- 
able conditions, make it possible that the first curvature of tendrils (see p. 471) is 
due to quick alterations in the turgor of the cortical parenchyma. If this be true, 
the turgor curvature is followed promptly by unequal growth, to which irreversible 
process the more permanent and more important of the tendril movements are due. 
Their behavior will therefore be discussed in connection with growth movements. 
Indeed it is not improbable that turgor changes underlie all such movements, though 
they are not apparent. 
In many plants whose leaves have no well-defined motor organs there exist slight 
modifications of structure looking in the same direction, with movements of less 
extent than those executed by well-developed motor organs Moreover, there are 
to be found similar movements in young leaves that have no trace of motor organs, 
but these movements cease by the time the leaves have attained mature size. 
(Compare young and old leaves in figs. 686, 687.) Doubtless growth, that is, irre- 
versible changes in the size of the cells, as contrasted with the reversible changes 
produced by turgor, cause these movements. 
From the foregoing it is evident that no hard and fast line can be drawn 
between the displacements due to turgor and those due to growth. In fact there are 
all gradations between them. Therefore, the separate treatment must not be per- 
mitted to establish in the mind too sharp distinctions ; for distinctions are valuable 
chiefly as conveniences to the memory ; they have usually slight basis in nature. 
7. TROPISMS 
Growth curvatures. It is a matter of common observation that the 
various parts of a plant have definite positions. If they are mechani- 
cally displaced, the usual position often is resumed after a time by cur- 
vature. Again, if some external force acts upon them from an unac- 
customed direction, a curvature may result, restoring the customary 
relations so far as may be. Some of these curvatures have been con- 
sidered; namely, those that are due to changes of turgor. But a much 
larger number are due to growth, because few plants have such a struc- 
ture as to permit turgor variations to move an organ. On the contrary, 
every plant has some part where growth is either in progress or can be 
initiated, and consequently a curvature can be induced, if by appropriate 
mechanisms the amount or rate of growth can be modified locally. 
Practically all plants have such mechanisms, which are set into operation 
by various external stimuli. It will be most convenient to consider these 
tropic curvatures according to the stimulus that induces the reaction. 
Parallelotropic and plagiotropic organs. Observation shows that in 
certain plants the main axes respond to a tropic stimulus by placing them- 
selves parallel to the direction from which the stimulus acts, while other 
