196 PROCEEDINGS OP THE AMERICAN ACADEI\IY 



These papers show that there is a great uniformity in the character 

 and arrangement of the tissues composing the thuUus iu the different 

 genera. 



The adult stipe of the Laminariaceoe may be said, in general, to be 

 composed of four forms of tissue. At the centre is a bundle of twisted 

 and intertangled hyplui-like iilaments, among which are scattered long 

 trumpet-sliaped or lunnel-formed cells, placed by twos with their large 

 ends together. Wilt describes in detail a zone of tissue, composed of 

 sieve-tubes with very abundant sieve-plates, which surrounds this core 

 in the udult stipe of Maciocystis. Outside of the central filamentous 

 tissue, or luednUa, is a layer of elongated cylindrical cells, whose 

 side walls are very much thickened and abundantly marked with 

 pits, their intercellular spaces being filled by a slimy substance. In 

 AJacrocyslis and Lessonia, this layer, which may be termed tlie inner 

 cortex, is sharply defined against the medulla; but in Laminaria the 

 medullary filaments penetrate its inner cell-rows, and cause a blending 

 of the two tissues, so tliat no clear line of demarcation can be fixed 

 between them. Next without the inner cortex are rows of thin-walled 

 cells, more angular than the last in form. These compose a tissue called 

 the outer cortex. They become smaller toward the surface of the stipe, 

 and pass gradually into the single superficial layer of cells which forms 

 the epidermis. This latter layer consists of small cells whose exposed 

 outer walls are rounded and much thickened, and which contain the 

 olive-brown ))igmeiit characteristic of this group of Algte. The cells 

 of both outer cortex and epidermis are traveised by very delicate 

 partitions, both parallel and oblique to their walls. In Alaria, the 

 cortical tissue seems not to be distinguishable into outer and inner 

 layers. 



The lamina is, anatomically as well as morphologically, a terminal 

 expansion of the stipe, and shows the same tissues as the latter, but 

 in quite different proportions. The medulla resembles that of the 

 stipe, but the trumpet-shaped cells seen in the latter are not present. 

 Tiie cell-walls of the irmer cortex are thinner than in the stipe, and 

 contain no pits. The outer cortex forms but a small pait of the 

 thickness of the blade, and consists of small, thin-walled cells pass- 

 ing into an epidermal layer, as in the stipe. In Alaria, the stipe is 

 prolonged into the lamina as a midrib, whose anatomy is that of the 

 stipe itself. 



The ihizoids consist of a mass of cells somewhat thick-walled at the 

 centre, and decreasing in size and thickness of wall toward the sur- 

 face, tlie outer layer forming an epidermis. No trace of medulla is 



