THE ALGAE 



155 



Structure of the Shoot 



The centre of the shoot is occupied by very much elongated, rather 

 narrow central cells with fairly thick walls, which form an interwoven tissue. 

 These cells are united to one another at certain points where the cells 

 narrow and a thin cross-membrane is formed, leaving a tiny pit through 

 which cytoplasm is continuous. On each side of the pit is a small cap, 

 consisting of what appears to be coagulated cytoplasm. 



Connecting with these central cells are the collecting cells. They are 

 considerably shorter, and in passing towards the periphery of the thallus 

 they get progressively smaller. They are connected together by pits, with 

 or without protoplasmic caps, and form a network around the longitudinally 

 running central cells. 



On the periphery of the thallus are the assimilating cells, which form 

 rows parallel to one another but curved upwards and outwards at a constant 

 angle to the longitudinal axis of the whole shoot. These cells possess 

 numerous pits which are always destitute of protoplasmic caps. 



The whole thallus thus consists of a complex system of long and richly 

 branched filaments of which the assimilating cells form the ultimate branches. 

 Such a tissue is termed a plectenchyma. As a general rule division in these 

 filaments takes place at right angles to the longitudinal axis of each cell row. 



Growth takes place not only at the apex of the shoot, where cell division 

 is going on rapidly, but growth in width occurs at the apices of the assimilatory 

 cell rows, where new assimilatory cells are cut off" terminally, while the basal 

 cells of the assimilatory branch become collecting cells. Growth in length 

 may be produced by the elongation of the central cell, by the enlargement of 

 the conducting cells and by the further branching of the assimilatory filaments 

 to produce new rows among the pre-existing ones. 



Structure of the Attaching Organ 



The attaching organ does not show the division into separate zones that 

 is found in the shoot. 

 The cells nearest the sub- 

 stratum form a layer of 

 irregularly arranged cells of 

 varying shapes which fit 

 into the minute crevices in 

 the substratum (Fig. 143). 

 They have thick cell walls 

 and form an attachment 

 layer, three or four cells 

 thick. The remainder of 

 the attachment is composed 

 of rectangular cells in very 

 regularly arranged rows. 



Fig. 143. — Chondnis crispus. Holdfast at base 

 of stipe. (After Darbishire.) 



