June, 1913. 



KNOWLEDGE. 



203 



other types, for the cells are in connection with one 

 another by means of tine protoplasmic threads, 

 which connecting threads have not been detected 

 in other colonial forms of this group. These 

 genera may be arranged in a series as shown in the 

 diagram (see Figure 209). The evolutionary 



7&x(/or~t ntz 



Cf&f-ycfontona.s 



/Veodorina 



Figure 209. 



Vain 



Possible scheme of Evolution of 

 Volvox Line. 



d. Ulothrix. 



tendency represented by the base line may thus be 

 read as one leading to the association of motile cells 

 to form a free-swimming colony, and to the 

 elaboration and differentiation of the coenobium. 

 It appears that this line has led no further than to 

 Volvox, possibly owing to the instability of the large 

 body of loosely connected 

 cells. 



The manner of deriva- 

 tion of the filamentous 

 and thalloid types is by 

 no means so clearly indi- 

 cated by living forms, but 

 still a line can be traced 

 with a fair degree of 

 possibility through the 

 Tetrasporaceae. Essen- 

 tially, the filament is a 

 segmented tube, each 

 compartment representing 

 a single uni-nucleate cell. 

 Manyof the simpler forms, 

 Ulothrix, for example, 

 produce zoospores which are substantially of the 

 Chlamydomonas type, indications of the retention 

 of primitiveness in the reproductive condition. 

 Evidently if these filaments and plates have been so 

 evolved, there took place an increase in the length of 

 the resting periods, and an association of the resting 

 cells to form a chain or fiat plate. A few examples 

 from the Tetrasporaceae will show that there is 

 certainly some evidence for such a process. 



In Chlorangiuin the swarm-spores attach them- 

 selves, develop a short stalk, and divide to form 

 tree-like growths of which an)- one cell may detach 

 itself and act as a bi-ciliate swarmer. The signifi- 

 cance of this genus is rather the marked non-motile 

 stage in the life-history. In Hormotila the spherical 

 cells are enclosed in cylinders of mucilage, and the 

 whole association shows some approach to the 

 filamentous condition. Each cell can produce 

 zoospores by division. Radiofilum is still nearer to 

 the filamentous condition (see Figure 210, c). None 

 of these genera is at all common. On the other 

 hand Tetraspora, a somewhat common alga, builds 

 gelatinous aggregations of indefinite shape, the 



Figure 210. Some types of Ulothrix and Ulva Line, 

 a. Chlorangium. b. Hormotila. c. Radiofilum. 



spherical cells being arranged in groups of four in 

 the mucilage. Monostroma is somewhat similar, but 

 forms a definite plate one cell thick, and on account 

 of its thalloid type of body is generally placed in the 

 Ulvales. These two lead to the genus Ulva, a 

 common green seaweed, where the thallus is quite 

 definitely two-layered and more highly differentiated. 

 A general tentative scheme for the evolution of 

 simple filamentous and plate-like forms is shown 

 diagrammatically in Figure 211. 



From the simple Ulothrix type a series showing 

 various higher degrees of differentiation may be 

 traced in the plexus of orders called the Ulo- 

 trichales, but the consideration of these would lead 

 us into a mass of details rather to be avoided in 

 a purely general outline. 



The third main line has probably led to the so- 

 called coenocytic type of thallus, where the plant 

 body is unseptate or nearly so, but each " cell " thus 

 formed is multi-nucleate. The living forms in the 

 Chlorococcineae, none of which is very generally 

 known, however, seem to lie near the root of the 



siphoneous type. With- 

 out going into detail, then, 

 it may be said that 

 Pediastrum (a common 

 moorland alga) and 

 Hydrodictyon (the beau- 

 tiful, but rare, " water- 

 net ") are really colonies 

 of many multi - nucleate 

 " cells." Protosiphon is 

 obviously a fairly primi- 

 tive member of the 

 Siphonales, possessing a 

 hollow, sac-like coenocytic 

 body. The more highly 

 differentiated members of 

 this group will not be 

 considered, but it may be interesting to note that 

 many of them, like Caiderpa or Acetabularia, are 

 beautiful forms inhabiting the warmer seas. 



It is obvious that our present flora must represent 

 very many lines of descent, which have branched 



ULOTRICHALCS 

 V/of-An? 



e. Tetraspora. f. Monostroma. 



7&ufroA/t<m 



Ho 



af'aa 



Tef-rotpora 



ULVALE0 

 „> Monoftroma- 



CAloranocufn 



\ 



Ch la *»ydo i*\o«a<( 

 Ancestor 



Figure 211. 



Possible scheme for Ulothrix and 

 Ulva Line. 



