456 TliANSACTIONS AND PKOOEEDINGS OF TIIR [sess. r.xviii. 



it. This shrinkage is, however, in most cases easily allowed 

 for, as it is naturally greatest at the base, the oldest part, 

 of the rootlet. 



So far, the position of primary Lenticels alone has been 

 discussed, but as the root increases in thickness, secondary 

 structures of the same kind arise at apparently indeterminate 

 points of the surface. 



It is difficult to obtain any clue to the causes underlying 

 the formation of these organs, and though the determination 

 of their points of origin may possibly depend on accidental 

 rupture of the cork covering, and the formation of new 

 initial areas beneath the old phellogen, this is only a sug- 

 gestion with hardly any evidence to support it. 



Apart from the effect of pressure on the initial cells, 

 another partial cause of lenticellar formation is to be found 

 in excess of moisture, either in the form of water or of 

 moist air. 



The best evidence of this is perhaps to be found in the 

 hypertrophy of lenticellar organs in moist situations. 



When a stem or root bearing Lenticels is placed in water 

 or suspended in a saturated atmosphere, the division of the 

 lenticellar initials often becomes so accelerated, and the 

 cohesion of the complementary cells so increased, as to give 

 rise to the formation of long worm-like outgrowths of 

 iBrenchymatous tissue, which may easily be mistaken for 

 roots, though undoubtedly true roots may arise beneath the 

 Lenticels at a later period. 



The numerical proportion which the Lenticels on the 

 lower side of a branch bear to those on the upper varies 

 within somewhat wide limits, as the following numbers 

 taken from Devaux will show : — In a three-year-old shoot 

 of Lime there were 6*7 Lenticels per square centimeter on 

 the upper surface and 7'7 on the lower, while in a one-year- 

 old shoot of Alder, the upper surface bore 9 '7, the lower 

 10 "8, in the same area. It is interesting to notice that a 

 three-year-old branch of the same tree had 4 "6 on the upper 

 as compared with 3 '7 on the lower. 



These numbers do not indicate any very marked difference, 

 and, in ascribing that which does exist to differences of 

 moisture, it must be borne in mind that in many cases the 

 growth in thickness of a branch is greater on the upper 



