484 BOTANY 



Nature of Stimuli 



Stimuli may be either external or may develop within the protoplast. 

 The latter are said to be autonomous, and arise from changes within 

 the protoplast itself. Autonomous stimuli are usually much less 

 readily determinable than external ones, but this is not always the 

 case. For instance, the shape of a growing organ may be manifestly 

 due, in part, to pressures arising automatically in the course of 

 development. Stimuli may be thermal, chemical, mechanical, photic, 

 or electric. 



Movements of Cellular Members 



The movements shown by the organs of the higher plants are of 

 two kinds, movements of growth and movements of variation. 



Movements of Growth 



In an actively growing shoot or root, the apex is moved upward 

 (or downward) by the rapid elongation of the growing zone of tissue. 

 The rate of growth is by no means uniform, and a familiar experi- 

 ment readily shows this. The root of a seedling is carefully marked 

 with a series of transverse lines, placed at equal intervals, and 

 examined after a given time. It is then found that the rate of 

 elongation shown by the separation of the marks has been very 

 different at different points (Fig. 456). 



During its earlier phases of development the growth of an organ 

 is usually slow, although the cell-division may be rapid. The growth 

 in extension then begins, and increases rapidly until a maximum is 

 reached, when it decreases until it finally ceases entirely. This is 

 especially well seen in the unfolding of leaves in the spring. The 

 leaf, within the bud, has all its parts indicated, and, except for its 

 minute size, is practically complete. With the unfolding of the bud 

 there is a sud'den increase in the size of the leaf, which within a 

 week or two may reach its full size, after which no further enlarge- 

 ment takes place. Even more remarkable is the very slow growth 

 of the leaves of many Ferns, which may require three, or even four 

 years for their complete development, but which for their unfolding 

 need but a few weeks, within which time they increase in bulk a 

 hundred-fold. 



While it is true that, in general, growth proceeds in a straight 

 line, it is found that the tip of a growing organ describes an orbit 

 about its axis. This " Circumnutation " is especially marked in 

 rapidly growing organs, such as tendrils and the tips of climbing 

 shoots. The path traced by the nutating apex varies from a circle 

 to a straight line due to simple oscillation to the right and left. As 



