Chap. II. BREAKING THBOUGH THE GEOUND. 91 



if this is so, we must admit from the cases just given, 

 that a tendency in the upper part of the several 

 specified organs to bend downwards and thus to be- 

 come arched, has no v become with many plants firmly 

 inherited. The arching, to whatever cause it may be 

 due, is the result of modified circumnutation, through 

 increased growth along the convex side of the part ; 

 such growth being only temporary, for the part always 

 straightens itself subsequently by increased growth 

 along the concave side, as will hereafter be described. 



It is a curious fact that the hypocotyls of some 

 plants, which are but little developed and which 

 never raise their cotyledons above the ground, never- 

 theless inherit a slight tendency to arch themselves, 

 although this movement is not of the least use to 

 them. We refer to a movement observed by Sachs 

 in the hypocotyls of the bean and some other Legumi- 

 nosse, and which is shown in the accompanying figure 

 (Fig. 59), copied from his Essay.* The hypocotyl 

 and radicle at first grow perpendicularly downwards, 

 as at A, and then bend, often in the course of 24 hours, 

 into the position shown at B. As we shall here- 

 after often have to recur to this movement, we will, for 

 brevity sake, call it " Sachs' curvature." At first sight 

 it might be thought that the altered position of the 

 radicle in B was wholly due to the outgrowth of the 

 epicotyl (e), the petiole (p) serving as a hinge ; and 

 it is probable that this is partly the cause ; but the 

 hypocotyl and upper part of the radicle themselves 

 become slightly curved. 



The above movement in the bean was repeatedly 

 seen by us ; but our observations were made chiefly on 

 fliaseolus midtijlorus, the cotyledons of which are like- 



' Aibeiten doa bot. Instit. Wiirzburg,' vol. I 1873, p. 403. 



