Trentepohliaceas 



307 



the lamellae of the cell-wall are parallel and the wall at the free end of the 



apical cell is of approximately the same thickness as 



the rest of the cell-wall. They are, however, developed 



in varying degree in those species in which the lamellae 



of the cell-walls are divergent. The fewer the lamellse 



at the growing extremity of the apical cell, the more //// \\\\ .1 



complete is the development of the apical cap (West 



& Hood, 11). 



The apical cap is a secretion of the apical cell and is a 

 bonnet-shaped mass of pectose fitting firmly over the delicate 

 end-wall. It is at first homogeneous, consisting of one layer, 

 but in older cells it is composed of two or more layers (fig. 

 201 A and (?), and sometimes apical cells are surmounted by 

 a successive series of apical caps (fig. 201 B). Injury or removal 

 of the cap appears to be followed by the rapid formation of 

 another one. In some species, such as T. aurea, the apical cap 

 may become so cumbrous as to be an impediment to apical 

 growth, in which case it is usually displaced by a slight change 

 in the direction of growth of the apical cell (fig. 202 B). The 

 original cap is therefore often left in a lateral position some 

 distance behind the growing apex, which develops a new cap. 

 As the growth of the branch proceeds a repetition of this 

 lateral displacement of apical caps results in branches with 

 several lateral excrescences (West & Hood, '11) ; fig. 202 C. 



In the division of the apical cell the transverse wall arises 

 as an annular ingrowth from the middle region of the lateral 

 walls. In some species the transverse walls are lamellose like 

 the lateral walls. 



Wildeman ('99) has shown that injuries to the thallus of 

 Trentepohlia and Phycopeltis result in rapid regeneration. 



Fig. 200. Semi -diagram- 

 matic figure of the 

 structure of the wall of 

 the apical cell of T. 

 Montis-Tabulw (Eeinsch) 

 De Toni var. ceylanica 

 W. & G. S. West. The 

 innermost layer of cellu- 

 lose (I) is filled in black. 

 ap. cp., apical cap. (After 

 West & Hood.) 



There is normally one nucleus in each cell, but in old cells several nuclei 

 have sometimes been detected. There are several disc-shaped or band-shaped 

 chloroplasts in each cell, without pyrenoids. The green colour is in some 

 species masked by haematochrome, some species of all the genera having an 

 orange or red colour. The large quantity of hsematochrome probably acts as 

 a screen to the chloroplast against too intense light. Trentepohlia cyanea 

 Karsten ('91) is bluish in colour and Phycopeltis nigra Jennings ('96) is 

 black, but in these and certain other forms the unusual colour is due to the 

 impregnation of the cell-walls with some dark pigment. 



Reproduction most commonly occurs by zoogonidia, which are liberated 

 from specially differentiated zoogonidangia. These are solitary or developed 

 in clusters, and are terminal, lateral, or intercalary in position. The terminal 

 zoogonidangium is formed from the apical cell and in consequence the growth 

 of the branch is temporarily arrested. After the escape of the zoogonidia 



202 



