234 ELEMENTS OF BOTANY 
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, 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 fre- 
quently the mode of reproduction is by means of spores, new cells 
(usually four in number) formed inside one of the older cells (ascus). 
At length the old cell-wall bursts and the spores are set free to begin 
an independent existence of their own. 
In examining the yeast-cell the student has been making the 
acquaintance of plant life reduced almost to its lowest terms. The 
very simplest plants consist of a speck of jelly-like protoplasm. 
Yeast is more complex from the fact that its protoplasm is sur- 
rounded by an envelope of cellulose, —the cell-wall. 
303. Fungi. — The yeasts, molds, rusts, mildews, and 
mushrooms represent an immense group of plants of which 
about forty-five thousand species are now known in the 
world. They range from the very simple to quite com- 
plex forms, growing as saprophytes or parasites under 
a great variety of conditions. Their structure and life 
history are so varied as to constitute a long series of divi- 
sions and subdivisions.1. Chlorophyll is absent from fungi, 
and they are destitute of starch, but produce a kind of 
cellulose which appears to differ chemically from that of 
other plants. Unable to build up their tissues from car- 
bonic acid gas, water, and other mineral matters, they are 
1See Strasburger, Noll, Schenck, and Karsten’s Tes/-Book of Botany, 
pp. 375-421, incl.; also Potter and Warming’s Systematic Botany, p. 1, and 
Engler’s Sylubus der Pflanzenfamilien, Berlin, 1907, pp. 25-51. 
