1919 Child; on Susceptibility of Algae 259 



differences in susceptibility of main and lateral axes are similar to those 

 observed with other agents. 



In various species the course of death along the axis in standing water 

 was observed and was found to be basipetal like the other susceptibility 

 gradients, provided the axes were not too closely crowded. When they were 

 crowded, regions where different axes lie in contact are likely to die earlier 

 than would be expected from their level in the axis, a fact which suggests 

 that under these conditions lack of oxygen may be a factor in death. The 

 death gradients were observed in standing water in Desmarestia, Agard- 

 hiella, Euthora, Bonnemaisonia and Callithamnion. The criterion of death 

 used in these cases is the change in color in consequence of loss of the plant 

 pigment. It is of interest to note that the course of death under these con- 

 ditions is similar to that observed with the various agents used. 



Nereocystis and Callymenia 



Among the species examined which show peculiarities either in growth- 

 form or in other characteristics, the kelp Nereocystis luetkeana is first con- 

 sidered. It has already been shown that in this species the stipe and the 

 frond differ as regards the position of the chief growing region, that of the 

 stipe being at the upper (apical) end (Sheldon, 1915), while that of the 

 frond is near the basal end (Fallis, 1915). In my susceptibility observa- 

 tions young complete plants ranging in length from 30—150 cm. and separ- 

 ate stipes and fronds up to 100 cm. from more advanced plants were used. 

 In HCl m/200 a change in color from the normal to a dirty green, also a 

 decrease in turgor of the cells begins in about one hour. In the stipe the 

 changes appear first 1-3 cm. below the float and progress rapidly in both 

 directions, i. e., toward and over the float and basipetally over the upper 

 14 — % the length of the stipe and from that point on more slowly to the 

 base. In these young plants the float is somewhat less susceptible than the 

 upper region of the stipe, but the difference is not very marked. The upper 

 y^ — Ys of the stipe, the region where growth is occurring most raj^idly, is 

 much more susceptible than more basal levels. 



In the frond the change in color and loss of turgor begin at or within 

 a few millimeters of the base of the flat portion at about the same time as 

 in the upper portion of the stipe and progress rapidly along the frond for 

 a distance of 5 — 6 cm. in fronds 20 — 30 cm. long and over a greater distance, 

 e. g., 20 — 25 cm. in fronds 100 cm. or more in length. This region repre- 

 sents the most rapidly growing region of the frond. Progress of the death 

 changes farther toward the tip of the frond is slower, except in the distal 

 third more or less, the region of lowest susceptibility, where the differences 

 in susceptibility at different levels are slight. The short cylindrical con- 

 necting region between frond and float is somewhat less susceptible than 



