1919 Child; on Susceptibility of Algae 255 



naetabolic condition are associated and of the value of the susceptibility 

 method as an indicator of quantitative difference in physiological condition, 

 Jn this form the differences in susceptibility of the different apical regions 

 and axes afford a picture of physiological conditions in the different parts 

 of the plant which makes it possible to interpret to some extent the growth- 

 form in physiological terms. Many facts of experiment and observation 

 indicate that the inhibiting influence of a more active growing tip is effective 

 through a greater distance in the plant than that of a less active tip, and 

 when we find that the axes with higher susceptibility show alternate branch- 

 ing, those with lower susceptibility opposite branching we are able to assert 

 that the differences in growth-form are associated with quantitative differ- 

 ences in physiological condition, and second, that from what we know of 

 S'lsceptibility, these differences are in all probability differences in the rate 

 of fundamental metabolic reactions. 



Of course, the general thallus gradient in susceptibility, its differences 

 in different species, and the relations between these differences and different 

 growth-forms point to the same conclusions. Invariably, so far as observa- 

 tions have gone, it has been found that where main axes and lateral branches 

 are distinguishable in growth-form, they are also distinguishable as regards 

 susceptibility, and vice versa. However we may interpret the relation be- 

 tween susceptibility and metabolism, the significance of these facts cannot 

 be doubted. 



While no special attempt has been made to determine the susceptibility 

 relations of other than the vegetative parts of the plant, some observations 

 have been made on the susceptibility of cystocarps in Bonnemaisonia, Poly- 

 siphonia and Dasyopsis, and of the stichidia of Dasyopsis. In general the 

 cystocarps, or at least the earlier developmental stages, show a basipetal 

 gradient like the vegetative axes, and the earlier stages are more susceptible 

 than the later. The earlier stages of cystocarp development show about the 

 same susceptibility as vegetative axes from the same region of the plant, 

 but fully developed cystocarps are markedly less susceptible than the vege- 

 tative axes. The stichidia of Dasyopsis likewise show a distinct basipetal 

 susceptibility gradient. The earlier stages of stichidium development are 

 more susceptible than those regions of the thallus from which they arise 

 and also more susceptible than the later stages. In well developed stichidia 

 the basal half, more or less, is distinctly less susceptible than adjoining 

 vegetative regions. 



It is of interest to note that these specialized reproductive axes, the 

 stichidia, make their appearance first in the more basal regions of the 

 thallus, where the susceptibility is lowest, and from this region of first 

 appearance their region of origin gradually ascends the axis. Evidently 



