198 PROCEEDINGS OF THE AMERICAN ACADEMY 



plant often reaches a meter in length. This alga is an inhabitant of 

 the Arctic Ocean, and extends southward on the western shore of the 

 Atlantic to Boston, and in the Pacific to California and Japan ; being, 

 i;iic other Laminariacea^, a lover of cold waters. At Enstport, Maine, 

 it occurs at low-water mark, but south of that point it grows in deep 

 water, from which it is frequently washed ashore. 



At the suggestion of Dr. W. G. Fai-low, these investigations have 

 been undertaken with the purpose of learning the anatomy of the 

 adult and of the young frond, and the mode of formation of the per- 

 forations of the lamina, in Arjarum Tarneri. The material used has 

 been alcoholic, and has consisted of young fronds collected at Eastport 

 in September, 1877, by Dr. Farlow, and of adult plants obtained at 

 Marblehead, Mass., in November, 1885, by myself. 



1. Adult Anatomy. 



The stipe of the adult frond does not differ in its structure from 

 the general type of the Laminariacefe. The medulla and inner 

 corte*x together compose about one third of the whole diameter of 

 the stipe °at its base, but gradually enlarge so that, at the upper 

 end, they form one half of its diameter. The component threads of 

 the' medulla are rather loosely matted together, and have many 

 rounded cells scattered among them. These threads penetrate the 

 innermost layers of the inner cortex quite freely, causing a gradua.1 

 passage from one tissue to another, as in Laminaria. The cylindri- 

 cal cells of the inner cortex average from three to four times as long 

 as broad. Their side walls are much thickened and pitted, while 

 their end walls are thin. The outer layers belonging to this tissue 

 consist of smaller cells than the rest, and are cleaily distinguishable 

 from the inner rows of the outer cortex, which lie next to them. 

 These latter form a darker and more refractive layer of cells, since 

 they are very rich in protoplasm. The cells of the outer cortex 

 become smaller and thinner-walled toward the surface, and their super- 

 ficial layer is modified to form an epidermis. The epidermal cells 

 have their exposed ends rounded and much thickened, and contain 

 the characteristic olive-brown pigment. As in Laminaria, the cells of 

 the whole outer cortex are crossed by very delicate parallel and oblique 

 walls. The epideimis and cells immediately below present all the 

 appearances of growing tissues. 



The midrib continues the tissues of the stipe, but the ratios between 

 the different tissues vary markedly from those in the stipe. The outer 



