96 BIOLOGY 



definite functions, so that an organ contains several tissues 

 grouped together to form a complex structure. 



In the study of multicellular organisms, which follows in the 

 later chapters, it will be seen that some of them have only a 

 few simple organs, while others have many complex organs. 

 Those which are of simple structure and have few organs we 

 call low organisms, while by high organisms we refer to those 

 whose structure is complex. 



PEN1CILLIUM, A SIMPLE MULTICELLULAR PLANT 



As an example of a multicellular plant with very slight 

 complexity, we will study one of the common molds, which 

 may be found growing upon almost any moist food the world 

 over. It may usually be obtained in abundance by placing 

 a bit of bread or a slice of lemon in a dish, covering it so that 

 it will be kept from drying, and allowing it to remain in a 

 warm place for a few days. The object will soon become 

 covered with a mold (Penidllium) which after a day or two 

 assumes a greenish-blue color. This organism is somewhat 

 difficult to study under the microscope because it is so massed 

 together that special methods have to be taken for preparing 

 the specimens. The best method is to plant some of the spores 

 upon a little jelly which has been hardened on a glass slide, 

 and then study the spores under the microscope every day 

 and notice the method by which they sprout and eventually 

 form the complete plant. 



Structure. The structure of Penidllium may best be under- 

 stood by studying Figure 42. It is made up of a mass of deli- 

 cate, branching threads, extending in various directions. These 

 threads are white or colorless and very minute. In the com- 

 mon species of Penidllium they are hardly visible to the naked 

 eye, although in some species of molds they are slightly larger, 

 and in others they are large enough to be plainly seen. These 

 threads, which are known as the mycelium (Gr. mykes = fun- 

 gus), have the function of assimilation, and absorb nourish- 



