CELLULAR STRUCTURE OF ORGANISMS 99 



widely scattered are these floating mold spores that it is hardly 

 possible to expose any moist organic substance even, for a 

 few minutes, without its becoming inoculated with some of 

 them and showing, a few days afterwards, the growth of molds 

 upon its surface. 



Penicillium has a second method of multiplication which is 

 rarely seen. It occurs only under special conditions which are 

 not understood, and it has not been observed by many bota- 

 nists. It consists in the formation of minute sacs, within which 

 spores are formed, usually four or eight in number. These 

 sacs are known as asci and the spores are ascospores. Even- 

 tually the sacs burst, the spores come out and are then capable 

 of developing into new plants. This method of forming spores 

 is evidently similar to that already described in Yeast (see 

 Fig. 32 s), and shows that yeast is closely related to the 

 molds. The same method of spore formation is found in a 

 large number of other plants (lichens, cup fungi, etc.) and is 

 used as a basis of classification for a class of Fungi called As- 

 comycetes (Gr. ascus = sac + mykes = fungus). It must be 

 noted, however, that not all of the molds form spores in this 

 way. The one shown in Figure 42 D has a method of repro- 

 duction by conjugation. 



OTHER SPECIES OF MOLDS 



Molds are very abundant in all parts of the earth wherever 

 there is much moisture, and any bit of organic material left 

 to itself will be sure to show signs of their growth in course 

 of tune. Many species of molds, which to the naked eye 

 closely resemble each other, may be distinguished by careful 

 microscopic study. In all cases the plant is a branching, 

 colorless mycelium, similar to that described in Penicillium. 

 In a few species, however, the mycelium is not divided into 

 cells by partitions, as in Penicillium, but the whole thread 

 forms one continuous mass called a syncytium; Fig. 42 E. 

 The chief method by which the molds are distinguished from 



