78 



BOTANY 



PART I 



limited growth, and serve as leaf -like ORGANS OF ASSIMILATION. 



These short shoots may 

 again exhibit a division of 

 labour among themselves. 

 Such forms are of the 

 greatest interest morpho- 

 logically, as they show how 

 the leaves of cormophytes 

 could have arisen from short 

 shoots. 



Leaf- like short shoots have 

 evidently arisen independently in 

 a number of series of thalloid 

 plants. These organs, serving 

 for assimilation, have all as- 

 sumed similar leaf-like forms. 

 Thus the leaf -like branches of 

 the Siphoneae and Brown Algae 

 are not homologous with those of 

 the Red Algae but only analogous. 



8. Internal Structure 

 of the Thallus. Thalli, 



whether segmented or unseg- 

 mented, may consist of a 



FIG. 86. False branching in Cyanophyceae. A, Plecto- single protoplast (e.g. SipllO- 



nemaWollei; only the upper end of the broken filament neae Cdulerua Fio\ 346) Or 



grows out as a branch. B, PI. miraUle; both ends r 11 1 ' T i i 



proceed to grow. (OLTMANNS after KIRCHNEB and Ot man y Cells - lu fcne latter 



BORNET.) case the cells are arranged 



in filaments (Fig. 84), sur- 

 faces, or are united to form a cell mass. The simplest multicellular 

 thalli are composed of uniform 

 cells all capable of division. 

 As soon as a growing point is 

 defined a distinction between 



MERISTEMATIC and PERMANENT 



cells is apparent. The extreme 



tip of the apical growing point 



is nearly always occupied by a 



single cell termed the APICAL 



CELL. This often differs little 



from the other cells, as in the 



case of Cladophora glomerata 



(Fig. 84). The dome-shaped 



apical cell is prominent on 



the multicellular long shoots of the Brown Alga, Cladostephus verti- 



cillatus (Fig. 89). 



FIG. 87. Portion of the mycelium of Penicillium. 

 ( x about 35.) 



