HOW TO KNOW THE SEAWEEDS 



It is often necessary to examine the apex of thallus branches to de- 

 termine the manner of growth. The apex, also, is often so relatively 

 deUcate that it permits observation of cellular structure which is ob- 

 scured in the denser, older parts of the thallus (See Fig. 18). Growth 

 may occur in a variety of ways. In some uniseriate filaments, growth 

 may proceed by intercalary division of cells in any part, or in a 

 special region of the filament (See Fig. 106). In trichothallic growth 

 such intercalary divisions occur in multiseriate thalli at the base of one 

 or more apical hairs (See Fig. 19 and glossary for explanation of these 

 terms). Apical growth may occur in other ways, namely, by means 

 of a single apical cell which cuts off new cells from below, or by a 

 small or large number of cells forming a "fountain type" of apical 

 meristem (See Fig. 20), each cutting off cells from lower sides. Still 

 another manner of growth is observed in many complanate thalli in 

 which the actively dividing, meristematic cells lie along the margin 

 so that growth proceeds by expansion of the niargins. 



Fig. 8. Examples of various kinds of holdfasts. A. A simple, modified, basal cell 

 (Chaetomorpha); B. Unicellular rhizoids (Lophosiphonia); C. A solid basal disc (Grin- 

 nellia); D. Creeping stolons with adherent discs (Gelidium); E. A. horny cone (Sargassum). 



18 



