IRRITABILITY. 421 



The mode of branching, the angle which the branches 

 make with the primary shoot or root, if it persists, and with 

 each other, is a very characteristic feature in the habit of 

 a plant. We naturally assume that light and gravity must 

 exercise a considerable influence in determining the form 

 of branching of a plant, but we are now principally concerned 

 with the question as to whether or not there is any inherent 

 tendency which constitutes to determine the form of branching 

 which any particular plant presents. 



In the course of his observations upon the " Mobility of 

 Plants," Dutrochet was led to consider the relation of branches 

 to the axes bearing them, and he came to the conclusion, on 

 grounds which we shall become acquainted with hereafter 

 when we are considering the relation between the direction 

 of growth of shoots and roots to the plane of the substratum 

 bearing them, that the relation between axis and branches is 

 of such a kind that the angle between them, in so far as it is 

 not modified by external influences, is uniformly a right 

 angle. It does not appear, however, at least as far as roots 

 are concerned, that the angle in question is necessarily a 

 right angle. In his researches upon the growth of the roots of 

 seedlings, Sachs established the following facts concerning 

 this angle, the proper angle as he terms it : the lateral roots 

 which arise near the apex of the primary root make an acute 

 angle with the acroscopic portion of it : those which arise 

 near the base of the primary root are nearly horizontal, so 

 that their proper angle is approximately a right angle : those, 

 finally, which arise at the junction of the root with the hypo- 

 cotyl, or from the hypocotyl itself, form an obtuse angle with 

 the primary root and an acute angle with the hypocotyl, so 

 that in this case the proper angle is greater than a right angle. 

 The directive influence of the parent-axis upon the branch is 

 not, then, as Dutrochet thought, simply an instance of the 

 relation existing between an organ and its substratum, but it 

 is a manifestation of correlation of growth. The existence of 

 such a correlation of growth between axis and branch is 

 placed beyond doubt by the fact, which has been frequently 

 observed in certain species of Pinus and Abies, that if the 



