340 Pub. Piiget Sound Biol. Sia. Vol. 2, No. 53 



There is a sharp line of demarcation between the cortical growth of 

 two seasons, which gives the appearance in section of annual rings (Plate 

 59^ Fig. 7). This is due to the fact that the cells first formed in the 

 spring elongate much more than those formed later in the season, while 

 those cells which form the outer cortical layer of the resting season are 

 small and isodiametric, and develop heavier walls. 



d. Desmarestia lierhacea (Turner) Lamouroux 



(Plate 54, Fig. 3; Plate 60, Fig. 1-5). 



Neither young plants nor jolants bearing assimilating hairs were col- 

 lected, hence it has been impossible to determine the process of development 

 of the various tissues. However the tissues of the mature plant in the rest- 

 irg condition resemble so closely those of D. ligulata that it is natural to 

 suppose that they are formed in the same way. 



The increased width of the thallus of D. lierhacea over that of D. ligu- 

 lata is due to the fact that all primary growth, in older parts of the plant, 

 takes place at the edge of the thallus. The cortical cells in this region 

 divide parallel to the surface of the thallus, and the inner cells develop into 

 ground tissue, while the superficial cells divide rapidly at right angles to 

 the surface to accommodate the increasing width. In the older region of 

 the lamina, nearer the midrib, the only growth is secondary, and consists 

 in further development of the conducting hyphae and sieve tubes. 



There are five definite tissue regions in the thin flat lamina of the 

 thallus, the two cortical layers, each underlaid by the large cells of the 

 ground tissue, and between the two ground tissue regions, forming the 

 central laj'er, a weft of interwoven conducting hyphae, among which ramify 

 the branching sieve tubes (Plate 60, Figs. 1, 2, 3). 



n is in the increased development of the sieve tubes of the secondary 

 inner assiniilation tissue that Z). herhacea differs most materially in struc- 

 ture from Z). ligulata. The secondary branches of the lateral veins, which 

 originate as in T>. ligulata, continue their growth among the conducting 

 hyphae of the central layer of the lamina, branching and rebranching, 

 Avithout the investment of small inner assimilation cells with which older 

 parts of the system are provided, but themselves containing many chroma- 

 tophores, as do all young cells of the primary thallus. They are easily 

 distinguished from the conducting hyphae, many of which are of about the 

 same diameter and also contain chromatophores, since in sections lightly 

 stained in Bismarck brown, the ring of callose which is forming about the 

 edge of the sieve plate takes the stain much more deeply than do the cell- 

 ulose walls (Plate 60, Fig. 1). 



