352 STRUCTURE AND LIFE HISTORIES 



ized into spores, which escape from the cell-wall of the 

 mother-cell. In a number of many-celled plants (e.g., 

 Ulothrix) practically every protoplast has the capacity of 

 becoming organized into one or more spores which escape 

 from the old cell-cavity. The next higher step is the 

 restriction of spore formation to certain cells in special 

 organs (sporangia), while the other cells function only 

 vegetatively. 



Fig. 261. — Woodwardia orientalis. Portion of a leaf bearing numerous 

 young plantlets on its upper surface. 



319. Cell-fusions. — Through all the variations of re- 

 production by spores there is, as a rule, only the separation 

 of protoplasts from the parent body, never a cell-fusion or 

 nuclear-fusion. Some plants, however, such as Ulothrix 

 (Fig. 262), have been found to produce two sizes of 

 spores, and the small spores must always unite before 

 they can develop into full-sized, mature individuals.^ 

 Attention has already been called (Chapter XVIII) to 

 the condition of similar sized gametes (isogamy)', as in 

 Spirogyra, in contrast to that of unequal gametes {hetero- 

 gamy), as in Ascophyllum and Fucus. 



' In certain cases {e.g., Ulothrix) the microspores may develop small, 

 imperfect individuals without fusion. 



