254 Pub. Puget Sound Biol. Sta. Vol. 2, No. 48 



lated with the growth-form of the species is significant. The following case 

 of Ptilota pectinata is discussed somewhat more fully as illustrating the 

 significance of the relation between susceptibility and growth-form. 



In the thalli of Ptilota pectinata examined the branching of the prim- 

 ary axes is at first alternate in a single plane, but later a second branch 

 arises opposite each of the first branches. These branches of later origin 

 are more susceptible than the older branches opposite them. The secondary 

 branches on the primary lateral branches, or at least those of later origin, 

 are opposite, but when a lateral branch becomes converted into a main axis 

 by increasing distance from the primary growing tip or by injury to or 

 removal of the latter, it changes its form of branching from opposite to 

 alternate, and this is afterward again transformed into the opposite type by 

 the later appearance of a branch opposite each of the earlier branches. In 

 general the apical regions of main axes with alternate branching are more 

 susceptible than those of lateral branches with opposite secondary branch- 

 ing. Moreover, a general thallus gradient appears among the lateral 

 branches on a single axis, those nearer the apical end being more susceptible 

 than those farther basal. 



Here the differences in growth-form and in susceptibility confirm and 

 supplement each other. The apical regions of the main axes are in different 

 physiological condition from those of the doubtless somewhat inhibited 

 lateral branches, and all the facts, including the differences in susceptibility 

 (see discussion below) indicate that the difference is primarily a difference 

 in rate or intensity of fundamental metabolic reactions, the apical regions 

 of the main axis possessing a higher rate than those of lateral branches. If 

 this conclusion is correct, it follows that in these regions of higher metabolic 

 rate branches can arise only alternately, doubtless because each new branch 

 or more specifically its growing tip, inhibits the development of other grow- 

 ing tips within a certain distance. Later, however, when the branch has 

 attained a certain length and the distance between this tip and the axis from 

 which it arose has increased to a certain amount and perhaps also the grow- 

 ing tip of the branch has become physiologically older and less active, a new 

 branch arises opposite. 



Again, on the primary lateral branches which are evidently somewhat 

 inhibited in their growth-activity by the growing tip of the main axis, the 

 secondary branches are opposite from the beginning, because the lower 

 metabolic rate in these growing tips makes the range of their inhibiting 

 action on other growing tips less, i. e., their inhibiting action does not extend 

 so far as that of the more active tips. 



The case of this species is considered at some length because I believe 

 it affords a striking example both of the manner in which growth-form and 



