418 LECTURE XVII. 



the screw th. The weight, wt, counterbalances the box and anything it 

 may contain. 



In order to ensure the necessary uniformity of rotation it is necessary 

 that the centre of gravity of the box, B, and its contents should lie in the 

 axis of rotation. The centering is effected by altering the position of the 

 above-named movable brass plate, which lies in a chamber between in 

 and //, and which can be fixed in any requisite position. 



A great number of experiments, with the solution of this 

 problem as their object, have been made by Vochting. He 

 has found, in the case of peduncles, branches, primary shoots 

 and roots, that when these organs are withdrawn as far as 

 possible from the action of all external directive influences, 

 their axes of growth are nearly straight. In certain cases he 

 observed that organs which had been purposely induced to 

 curve, straightened themselves out during the course of the 

 experiment. The only at all satisfactory account is that 

 offered by Vochting, that growing organs possess a tendency,- 

 which he calls Rectipetality , to grow in a straight line, and so 

 far as anything is known about it at present, this tendency 

 appears to be inherent in them. 



But we have yet more to learn about the directions of 

 growth spontaneously assumed by growing organs. In order 

 to pursue this subject we must revert to the fact mentioned in 

 the first lecture (p. 9) that, with the exception only of plants 

 of the lowest organisation, the plant-body presents a distinc- 

 tion of a base and of an apex, and that the line joining the 

 base and the apex is the line along which growth in length 

 takes place (also p. 360). We will now further elucidate this 

 by reference to examples. Let us consider, first, the fila- 

 mentous body of an Oedogonium. One end of the body is 

 attached, the other is free ; the former is the base and the 

 latter the apex ; the line joining the two is the axis of growth. 

 The growth in length of the body is not, however, localised in 

 this case at either the base or the apex, but it elongates by 

 the growth of all its constituent cells; its growth is intercalary. 

 In a plant-body of somewhat higher organisation, such as that 

 of a Fucus, we find, as in Oedogonium, the distinction of a 

 base and an apex. We find, however, in a body of this kind, 



