320 REPRODUCTION IN PLANTS 



them will appear to be entirely empty. These cells 

 (heterocysts) have the function of breaking the filament 

 into shorter sections. As division of cells progresses a fila- 

 ment becomes longer and longer. But here and there 

 some cell turns into a heterocyst. This sort of cell is 

 attached loosely to the ordinary cells on either side of 

 it with the result that a slight disturbance, such as that 

 due to bending to accommodate growth, may easily break 

 the filament. The third type of cell is also considerably 

 enlarged but instead of being empty it is densely filled 

 with protoplasm and stored food. Its walls also become 

 thick and so changed chemically that they resist drying. 

 These are called resting spores because they do not 

 divide until after the other cells in the filament have 

 dried or frozen. This kills the ordinary cells but does 

 not kill the resting spores. On the reestablishment of 

 growing conditions the latter resume growth and cell 

 division and make new filaments. In this plant there 

 is seen to be a distinction between' growth and repro- 

 duction. Cell division produces growth but does not of 

 itself bring about reproduction. The differentiation of 

 some cells to form heterocysts does however reproduce 

 or multiply the number of plants during the growing sea- 

 son. The differentiation of the resting spores takes care 

 of reproduction following unfavorable weather conditions. 

 Obviously this arrangement has certain distinct advan- 

 tages to the plant for it makes more certain that the fila- 

 ments will be broken frequently and that reproduction 

 will occur after drought. 



Ulothrix (Fig. 80) is a simple filamentous green alga 

 found in fresh water streams and lakes. Each filament is 

 attached at one end to a stick or stone and free at the 

 other end. The basal attaching cell is specialized for the 

 purpose and the tip cell is rounded or sometimes pointed. 

 The remaining cells are at first all similar. Each makes 

 its own food, grows, and divides. When the filament has 

 reached a suitable stage of development there occurs a 



