1920 Pease; on Desmarestia 327 



serrate margins, and begin to appear not more than 2 or 3 cm, above the 

 holdfast. 



The mature parts of the frond are differentiated into several tissue 

 systems. The main axis, together with all its branches of all orders, is 

 traversed by a single row of cells, the axial filament or rachis, surrounded 

 by an inner assimilation tissue of small cells, the whole forming the so- 

 called "midrib." Surrounding this, and forming the main substance of the 

 broader parts of the thallus, is a ground tissue consisting of very large 

 cells, among which ramify slender filaments containing scattered chroma- 

 tophores. The whole plant body is covered by a cortical layer from one 

 to three cells thick. 



The entire young plant and the younger parts of old plants are 

 clothed during the active growing season in early spring, with opposite, 

 distichous rows of monosiphonous branching filaments, which give the 

 plant a most beautiful feathery appearance. The cells of the filaments 

 are crowded Avith chromatophores and according to Soderstrom (1889) can 

 be considered as assimilatory organs which have an important bearing 

 on the rapid development of the young twigs. 



According to Janczwski (1875), lateral and terminal filaments are the 

 same in structure and can be distinguished only by their position. They 

 are either simple or branched and the main filament and its branches de- 

 velop basipetally, the outermost cells being the oldest and in his opinion 

 the most vigorous. On account of the fact that one finds younger, shorter 

 branches among the older ones, Soderstrom reasons that the cells of the 

 rachis divide transversely and the new cells thus formed also send out 

 branches. The branches originate as opposite outgrowths from the upper 

 part of cells of the rachis. These become cut off from the parent cell by a 

 cross wall, elongate, divide transversely, and increase in length by re- 

 peated transverse divisions of the basal cell formed by their first trans' 

 verse division. 



Toward the end of tlie growing season the hairs break off just above 

 the growing region and are shed bodily. Their bases remain as small 

 thorn-like projections along the sides of the thallus twigs, gradually become 

 covered by the growth of the cortical cells, and persist in the form of 

 serrations or small thorn-like "short branches." 



The growing region, as in many other Phacophyceae, is intercalary, 

 lying between the thallus and the hair-like filaments, and giving off cells 

 to each. According to Jonsson (1901) the first two or three cell divisions 

 in the growing region are necessarily apical, but by subsequent divisions 

 of the inner cells the growing region becomes intercalar3\ The earliest 

 stages of development have never been observed, but the development of 

 the various tissues can be followed in the growing tips of the fronds. 



