31 4 METAMORPHOSIS 



quoted might be largely added to, still, on the whole, the morphogenetic influence 

 of gravity, much over-rated in HOFMEISTER'S time, is very limited. Gravity 

 has nothing like the effect on plant shape that light has. 



Growth in length, on the contrary, is in general markedly influenced by 

 gravity. Thus it has been clearly established that Cham and Phycomyces 

 (ELFVING, 1880 ; RICHTER, 1894) grow more slowly when inverted than when 

 in the normal position, and other plants behave in a similar manner. [HERING 

 (1904) has shown that this retardation of growth in consequence of inverted 

 orientation is a phenomenon of widespread occurrence.] When shoots and 

 roots are placed at an angle to the direction of gravity their upper and under 

 sides grow at different rates, with the result that these members bend in a way 

 which we shall have to study later (Lecture XXXIV). 



Growth in thickness of trees is also not uniformly affected by gravity. 

 In all sloping branches the upper side grows in thickness at a different rate 

 from the under. In Coniferae (and in Aesculus also) the under side grows more 

 in thickness than the upper side, while the converse takes place, at least at first, 

 in Dicotyledons (WiESNER, 1895-6). Extensive investigations undertaken by 

 HARTIG (1901) on the Coniferae show that the increased growth of the under side 

 may occur in the main stem also when it is placed horizontally even when ap- 

 propriately supported so that the action of mere weight on the under side is 

 neutralized ; the effect can be attributed under these conditions only to the direct 

 influence of gravity. Further, the under side exhibits not merely an increase 

 in secondary thickening, but also a special anatomical structure in the wood 

 formed ('redwood '), attributed also to the direct action of gravity by HARTIG. 



HARTIG further attempted to show that the increased growth and formation 

 of ' redwood ' arose from pressure longitudinally exerted on the cambium, 

 which must arise in general in horizontally-placed branches owing to the influ- 

 ence of mere weight. Similarly, feebler increase in thickness and characteristic 

 anatomical structure of the wood on the upper side may be accounted for by 

 the tension in the longitudinal direction to which the cambium is exposed in 

 that region. HARTIG has given us no direct proof of this conjecture, but no one 

 can deny that this explanation is an extremely probable one, for it has often 

 been observed that tension and pressure have a distinct influence on growth. 



Pressure exerted on growing cells must retard their growth and may finally 

 completely stop it. Cells so prevented from growing exert oh their part a pres- 

 sure on the surrounding tissues, often leading to quite obvious mechanical 

 results. As PFEFFER (1893) has shown, this outward pressure is brought about 

 in this way the cell-wall becomes relaxed through surface-growth, and the 

 whole osmotic pressure is thus directed against the external opposing layer. In 

 individual cases an increase in the osmotic pressure may also be observed under 

 such conditions. Frequently the plant is able in this way to overcome ex- 

 ternal resistance, as, for example, in the splitting of rocks by roots. 



Tension of necessity acts on cells in the reverse manner to pressure. An 

 increase in growth in the direction of the tension is to be expected, and that 

 such takes place is easily proved if a stem be stretched by a weight. The pull 

 has, however, at the commencement quite a different effect ; it acts as a stimu- 

 lus and induces a retardation in growth, followed later on by the acceleration 

 mentioned (HEGLER, 1893). 



The new cell-walls formed in cell division show many relations to tensions 

 and pressures exerted on them ; the planes of division appear parallel to the line 

 of action of the pressure and transversely to that of the tension, provided there 

 be no other circumstances to prevent that result (KNY, 1901). A further stimu- 

 lative effect of tension is seen in the mode of formation of the internal tissues. 

 In ripening fruits under natural conditions, and in experiments on other organs, 

 the amount and also the thickness of the mechanical elements increases in pro- 



