96 SUMMARY OF CURRENT RESEARCHES RELATING TO 



aflfectcd by tbc iufluence of light or gravity bo distinguiubcs under two 

 cases. The first is wlieu a curved organ is affected by eitber of tbeso 

 agents in a plane at rigbt angles to tbe plane of curvature ; ai)parent 

 or even actual torsions may tben result ; and tbese probably play an 

 important part in tbe mecbanics of climbing plants. Torsions may 

 also arise from ligbt or gravitation acting from one side on symme- 

 trical organs in sucb a way that tbe direction of gravity or of tlie rays 

 of ligbt does not coincide witb tbe i)lane of symmetry of tbe organ. 

 In tins way are brougbt about tbe beliotropic torsions of tbe leaf- 

 stalk of many plants. 



Geotropic Sensitiveness of the Apex of the Root.* — G. Firtsch 

 bas determined by experiment tbe two following laws on tins subject : 

 — (1) If placed vertically, roots of seedlings from wbicb tbe apex bas 

 been removed, grow as quickly, or nearly so, in lengtb, as those that 

 have not been decapitated. (2) Tbe roots of seedlings, when placed 

 in a horizontal position, if too large an extent of tbe apex bas not been 

 cut off, grow more quickly in lengtb than those that have not been 

 decapitated, but manifest no geotropic curvature. From these facts 

 he draws the conclusion that geotropic curvature is prevented by the 

 removal of that formative tissue which operates in the renewal of tbe 

 root-cap ; and that this cap-forming mcristem is therefore tbe tissue 

 which is geotropically sensitive. 



Deviation of Roots from their Normal Direction through the 

 Influences of Gases, j — For this phenomenon H. Molisch proposes the 

 term " aerotropism," and lays down the following laws as the result of 

 observation : — 



1. When a growing root is subjected on one side to the influence of 

 certain gases in sucb a way that the gas is presented for a considerable 

 time in unequal quantities on two opposite sides, the root deviates in 

 a definite way from its normal direction of growth. 



2. This influence of gases on growing roots bas been determined 

 in the case of oxygen, carbonic acid, chlorine, hydrochloric acid, 

 carburettod hydrogen, ammonia, chloroform, ether, and others. 



3. The roots are sensitive in different degrees to different gases ; 

 oxygen causes slight, carbonic acid more decided, and chlorine very 

 great deviations. 



4. If a gas acts too strongly on tbe root, it curves towards the 

 source of the gas (positive aerotropism), but if to a moderate intensity 

 only, away from it (negative aerotropism). The effect of oxygen is 

 somewhat more complicated. 



5. Positive aerotropism is tbe result of the concave side being 

 injured, and hence growing less rapidly in length than tbe opposite 

 side. It is difficult to explain why, in negative aerotropism, the side 

 exposed to tbe action of the gas should grow most rapidly. 



6. Decapitated roots display tbe same phenomena as uninjured 

 witb regard to aerotropism, but to a smaller extent. 



* Ber. Deutsch. Bot. Gesell., ii. (188i) pp. 248-55. 



t Anzeig. K. Akad. Wiss. WieD, 1884, p. 146. See Natuiforsclier, xvii. 

 (1884) p. 3G6. Also Ber. Deutsch. Bot. Gesell., ii. (1884) pp. lCO-9. Cf. this 

 Journal, iv. (1884) p. 772. 



