GROWING POINTS 17 



respective branches. A similar apical position of the growing 

 point usually obtains amongst the lower plants. 



A good example is afforded by the little Seaweed Sphacelaria, 

 which is not uncommon in rock-pools along the sea-shore. Here 

 each branch ends in a single large cell (Fig. 5, a), which consti- 

 tutes the growing point. During the active season of the year 

 this cell is constantly increasing in size, and, when it has attained 

 a certain length, it becomes halved by the formation of a cross- 



FIG. 6. Growing point (a) and adjacent part of the thallus of Dictyota. 

 Lettering as in Fig. 5. 



wall (Fig. 5, b). The upper portion again enlarges until a new 

 septum arises, whilst the lower half (termed a segment) under- 

 goes further division (Fig. 5, c-e) to form the cells of the mature 

 plant. The Seaweed Dictyota furnishes another good instance 

 of a unicellular growing point, whose mode of division will be 

 understood by reference to Fig. 6. The daughter-cells or seg- 

 ments in these two cases are cut off in a single series parallel to 

 one face of the cell. 



In most plants having a unicellular growing point the apical 



