102 BOTANICAL GAZETTE [AUGUST 
basipetally from cell to cell. In each bipinnate system of stem and 
branches death begins at the apex of the stem and the most apical 
branches, and proceeds basipetally in the whole system, that is, 
the branches nearest the apex of the stem are most susceptible, those 
lower down are less susceptible, and so on.to the base of the bipin- 
nate system. ‘The apex of the main stem of such a bipinnate system 
is highly susceptible like the apical branches, but the susceptibility 
decreases rapidly down the stem, so that at any level the stem is 
less susceptible than even the basal regions of branches at that 
level. These susceptibility gradients are remarkably regular and 
uniform with one characteristic exception. In the older more basal 
branches and in the stem of each bipinnate system of the thallus © 
it was observed that the basal cell or the two most nearly basal cells 
were usually somewhat more susceptible and died earlier than the 
next two or three cells apical to them. In other words, the gradient 
in such branches is basipetal except for one or two cells at the base. 
The greater susceptibility of these basal cells suggests the possi- 
bility that they are the seat of secondary growth activity in con- 
sequence of physiological isolation, as in Ectocarpus, but whether 
this is actually the case or not I do not know. It may be merely 
that these cells are subjected to greater mechanical stimulation 
than more apical cells as the branches bend with water movements. 
The greater susceptibility of more apical as compared with more 
basal branches in the same bipinnate system is undoubtedly due 
to the fact that the more basal branches are physiologically older 
and their rate of metabolism is lower than that of the more apical 
branches. | 
Pleonosporium Borreri.—Only very young plants of this species, 
consisting of a single unbranched axis of a stem with a few lateral 
branches, were examined. In every case a distinct basipetal sus- 
ceptibility gradient was found, both in the main axis and in the 
lateral branches when they were present. 
Ceramium rubrum and C. tenuissimum.—The susceptibility 
gradients in these two species are distinctly basipetal. The small 
cells of the extreme apical region are distinctly more susceptible 
than the larger differentiated cells below, but in the differentiated 
region the gradient is very regular for a distance of several milli- 
