230 ELEMENTS OF BOTANY. 



water, and other mineral substances, as ordinary green plants can. It 

 gives off no oxygen, but only carbonic acid gas, and is therefore to be 

 classed with the saprdphytes, like the Indian pipe among flowering 

 plante, § 151. 



283. Multiplication of Teast. — While yeast cells are under favorable 

 conditions for growth, they multiply with very great rapidity. Little 

 protrusions are formed at some portion of the cell wall, as the thumb of 

 a mitten might be formed by a gradual outgrowth from the main portion. 

 Soon a partition of cellulose is constructed, which shuts off the newly 

 formed outgrowth, making it into a separate cell, and this in turn may 

 give rise to others, while meantime the original cell may have thrown 

 out other offshoots. The whole process is called reproduction by budding. 

 It is often possible to trace at a glance the history of a group of cells, 

 like those of the right-hand cluster in Fig. 198, II, the oldest and largest 

 cell being somewhere near the middle of the group and the youngest and 

 smallest members being situated around the outside. Less frequently 

 the mode of reproduction is by means of spores, new cells (usually four 

 in number), formed inside one of the older cells. At length the old cell 

 wall bursts and the spores ^e set free, to begin an independent existence 

 of their own. 



In examining the yeast cell, the student has been making the acquaint- 

 ance of plant life reduced almost to its lowest terms. The very simplest 

 plants consist, like the slime-moulds, of a speck of jelly-like protoplasm. 

 Yeast is more complex, from the fact that its protoplasm is surrounded 

 by an envelope of cellulose, the cell wall. 



THE STUDY OF BLACK MOULD.i 



284. Occurrence. — This mould may be found in abundance on 

 decaying fruits, such as tomatoes, apples, peaches, grapes, and cherries, 

 or on decaying sweet potatoes or squashes. For class study it may most 

 conveniently be obtained by putting pieces of wet bread on plates for a 

 few days under bell jars and leaving in a warm place until patches of the 

 mould begin to appear. ^ 



1 RhizopuR nigricans. If any diflficulty is experienced in procuring material for 

 study, the common sage-green mould, Penicillium glaucum, can always be procured 

 and propagated as described in Huxley and Martin's Biology. 



' It wUl always be found much easier to obtain a good crop of the desired mould 

 by sowing its spores upon the wet bread that is used. Spores may be kept in- 

 definitely, in a dry condition, for this purpose. Exposing the bread to a confined 

 portion of the atmosphere of any place, e.g., a cellar, where the desired mould has 

 previously flourished will insure a prompt growth of the mould anew. 



