Cucurbita maxima and C. Pepo. 18 L 



From our prev'ous considerations, it is clear that the move- 

 ment of the tendril is but a normal manifestation of growth, 

 which is subject to the same influences as other vital phenom- 

 ena. It now concerns us to determine how this motion is pro- 

 duced. This we shall discuss under the following headings : — 



(1) Gi'owth in length. 



(2) Torsion. 



(3) Effect of irritation. 



(4) Circumnutation. 



(5) Spasmodic movement at the end of activity. 



(6) Coiling about a support. 



(7) Free coiling. 



Growth in length. — We have already seen that the tendril 

 arm begins to revolve as soon as it has unrolled from the bud 

 and becomes straight. At this time it is in a state of rapid 

 elongation through its entire length, and whiie the full length 

 ultimately reached must necessarily be largely determined by 

 the time at which it comes in contact with an object of support, 

 the arms generally aitain a length which is nearly or quite 

 twice that which they have when first unrolled from the bud. 

 It is therefore obvious that most rapid extension is simultaneous 

 with the greatest activity, and the two are directly correlated 

 throughout the entire period, of movement. It is therefore, to 

 this very rapid elongation that we must look in the first in- 

 stance, for a true explanation of the movement, while second- 

 arily, we must consider the differentiation of the component 

 tissues, particularly in their relations of mutual tension. 



The structure of the tendril, presenting as it does a diversity 

 of tissues, at once shows that this rapid elongation cannot be 

 partaken of by all the tissues in equal degrees. The fibro- vas- 

 cular bundles and the zone of wood cells are those elements in 

 which the least extension can occur, of all the tissues present. 

 With reference to all the other tissues, therefore, these elements 

 must be brought into a state of positive tension, which increases 

 in strenth as age advances and the component cells become 

 more strongly modified by the formation of secondary layers, 

 thus making them more resisting. 



The collenchyma tissue is also a structure which, while it may 

 be capable of greater extension than the elements previously 

 considered, is capable of comparatively little elongation after it 

 is once fully formed. It may exert a positive tension on the 

 wood cells and thus bring themselves into a state of negative 

 tension, but with reference to all tissues in a more active state 

 of growth, it is constantly in a condition of very strong, positive 

 tension. This may be at once seen on making sections. 

 Transverse sections are found to quickly bulge out in the center, 



