74 SUMMARY OF CUERENT RESEARCHES RELATING TO 



Passage from the Root to the Stem.* — E. Gerard concludes 

 from a careful examination of the facts connected with this subject 

 that a " collar " does not exist as a geometrical expression. Between 

 the root and the stem is a region, more or less extensive, where the 

 elements of the root, advancing to the higher parts of the axis, become 

 modified, gradually assuming the configuration, place, and importance 

 which they possess in the stem. The transformation of each of the 

 elements is independent of that of its neighbours, and may take place 

 slowly or very rapidly. Hence the collar, considered anatomically 

 from difterent points of view, presents the most variable aspects. The 

 transformation of the epidermal system furnishes no guide to the 

 limitation of stem and root ; the change in the epidermis is one of the 

 phases of the passage. 



Using the term in its widest sense, the collar may commence in 

 the upper part of the radicle and extend to the fourth internode, rarely 

 passing the cotyledons, or it may be entirely localized in the radicle ; 

 it may occupy a part of the organ, and the whole or a part of the 

 tigellum. Most often the passage is completely efiiected in the tigel- 

 lum. The size of the collar is in proportion to the diameter of the 

 plant. 



No family characters can be drawn from the study of the collar ; 

 its peculiarities are constant only in the species. It is connected 

 with the accommodation of the plant to its surrounding conditions. 



Causes of Eccentric Growthf — Dr. E. Detlefsen has investi- 

 gated the cause of eccentric growth in thickness of woody stems and 

 roots in a number of instances, and finds it attributable to the four 

 following causes : — 



1. Branches and axillary roots cause, at the point from which they 

 spring, a diminution of the tension of the bark, and consequently an 

 acceleration of the growth in thickness, which is most considerable 

 where the surface of the lateral organ forms the smallest angle with 

 that of the mother-organ. 



2. Every diminution or increase in the tension of the bark is 

 perceptible over a large extent in the longitudinal direction of the 

 bast-fibres. 



3. Every lateral pressure which causes curvature of the organs 

 brings about an increase in the tension of the bark on the side which 

 becomes convex, a diminution on that which becomes concave. 



4. Convex surfaces cause an increase, concave surfaces a decrease, 

 in the tension of the bark, which aftects chiefly the different sides of 

 curved branches and roots. 



These influences may be exercised either in conjunction or sej^a- 

 rately. 



Hydrotropism of Roots4 — The term " hydrotropism " has been 

 suggested for the tendency displayed by roots, when placed between a 



* Ann. Sci. Nat. (Bot.,) xi. (1881) pp. 277-430. 



t Detlefsen, E., ' Versuch einer mechanischen Erkliining des excentrischen 

 Dickenwachsthums verholzter Achsen u. Wurzeln.' 13 pp. (1 pi.) Weimar, 1881. 

 X Bull. Soc. Bot, France, xxviii. (1881) pp. 115-21. 



