102 



BOTANY 



The Plant-body. The Confervoideae show considerable range of 

 structure. The simplest forms (e.g. Conferva, Microspora, Fig. 71, C) 

 are unbranched cell-rows, the cells entirely similar. In other forms, 

 e.g. CEdogonium (Fig. 74), the unbranched filament is differentiated 

 into base and apex, the former attached by a special root or disk. 

 Branching filaments, e.g. Cladophora, Chaetophora, are common, and 

 sometimes, as in Draparnaldia (Fig. 71, D), the smaller branches con- 

 tain most of the chlorophyll, and constitute very simple assimilative 

 structures. Less commonly, as in Ulva and Coleochsete (Fig. 77), 

 the plant-body has the form of a flat thallus. 



Cell- structure. The cells usually have the protoplasm confined to 

 tha periphery, leaving a single large central vacuole, but sometimes 

 there are bands traversing this, and dividing it into more or less 

 complete chambers (Cladophora). A single nucleus, imbedded in the 

 peripheral cytoplasm, is found in most cases, but occasionally (Clado- 

 phora, Sphaeroplsea), the cells are multinucleate. There may be a 

 single large chromatophore, usually containing a single large pyre- 

 noid, or there are numerous chromatophores distributed through the 

 cytoplasm. The cell-wall may be thin and homogeneous, or it may 

 show more or less evident striation (Cladophora). The plants are 

 sometimes imbedded in a gelatinous matrix, such as is found in the 

 common genus Chaetophora. 



Cell-division. In multinucleate cells like those of the common 

 genus Cladophora, division-walls are formed without any preliminary 



nuclear division. In this case 

 the wall begins to form as a 

 delicate circular ridge of cellu- 

 lose projecting into the cell- 

 cavity. This ridge grows 

 toward the centre of the cell, 

 and finally forms a circular 

 division-wall which cuts the 

 protoplast in two. 



Where a single nucleus is 

 present, this divides in the 

 ordinary way before the divi- 

 sion-wall is formed. The latter 

 may form gradually, as in Cla- 

 dophora, or it may be formed 

 simultaneously. A curious 



F I a.72.-A,(Edoff nmmautumnale,Bhow. Codification of the ordinary 



ing a cell in process of division ; r, the cell-division is seen in the com- 



cellulose ring, which stretches to form mO n genus CEdogonium (Fig. 



the new cell-wall, r', in , which repre- rm\ ^^ i * ^i 



sents the same cell 15 minutes later; 72 > Here > before the cleus 



o, anoogonium (x500). divides, there is formed, near 



