16 



BOTANY 



meat, and has relatively little chlorophyll; the upper one alone 



divides further, and 

 furnishes the whole 

 of 



of the active cells 

 the plant. 



Branching filaments 

 are still more common 

 and occur in a great 

 many Algae and Fungi, 

 or even in the earlier 

 stages (Protonema) of 

 Mosses. The branches 

 may be all alike, or 

 there may be a main ax- 

 is with lateral branches 

 of different form ; the 

 latter are often e.g. 

 Draparnaldia, Batra- 

 chospernum, numer- 



FIG. 5. A, simple filament of CEdogonium sp. 



( X 300) ; r, the holdfast ; B, branching filament of OUS and crowded, and 

 Callithamnion floccosum ; sp, tetrasporangia. contain relatively larger 



chloroplasts than the cells of the principal axis, to which they 

 bear much the same relation that the leaves of an ordinary 

 shoot do to the stem. These much ramified lateral branches are 

 undoubtedly specially adapted to increase the area of green cells 

 exposed to light. 



Apical Growth. In most of the branching 

 filaments less often in unbranched ones 

 a further specialization is evident ; i.e. growth 

 from a definite apical cell (Fig. 6). In such 

 forms, except in the case of the formation of 

 a lateral branch, the ordinary cells do not 

 undergo fission after they are cut off from 

 the apical cell, which alone contributes to the 

 growth in length of the axis. 



The transition from the filament composed 

 of a single row of cells (Monosiphonous) to 

 the more complicated forms, where the axis 

 is composed of more than one cell-row, is very gradual. In the lat- 

 ter type, the segments of the apical cell, instead of remaining undi- 

 vided, divide longitudinally, so that each joint is composed of a 

 group of cells instead of a single one (Fig. 6). The further divi- 

 sions of the segments derived from the apical cell may result in 

 massive branching structures, such as characterize many of the 

 larger Red and Brown Seaweeds. In these massive forms it is the 



FIG. 6. Growing point 

 of Polysiphonia Woodii, 

 showing the apical cell, 

 x (X500). 



