FUNGI AND FUNGOUS DISEASES OF PLANTS 237 



Under magnification the hyphae may be seen to consist of 

 heavy, tubular cell walls, in which the granular protoplasm is 

 not separated into distinct cellular divisions by transverse 

 walls, as it is in most of the algae. 



Bread mold lives upon and within its nutrient substance 

 and absorbs food material directly from it. Parts that are in 

 contact with the sub-stratum do the work of food absorption. 

 Food is carried through the tubular cells to the parts of the 

 mycelium that are above the food material. Since nutrient 

 material is secured in ready and abundant supplies, the growth 

 and extension of the mold is usually quite rapid. 



224. Effect of mold upon bread. If a piece of bread upon 

 which mold is growing vigorously is kept moist, much of the 

 bread is consumed by the mold, but usually the mold will not 

 continue to grow until the bread is completely consumed. 

 Either because it has secured all the food it can extract from 

 the bread, or because it has secreted substances that prevent 

 its further growth, or because it is unable to hold its own 

 with other organisms (molds and bacteria), the bread mold 

 after a time ceases to grow. Other molds and bacteria may 

 appear, one kind following another for weeks, until the decay 

 of the bread is almost or quite complete. 



If the mold, and the material upon which it grows, is kept 

 tightly sealed, growth stops before all the food material is 

 used. Molds often grow for a time in jars of fruit, forming 

 upon the top of the fruit a coating which remains until the 

 jar is opened. If this coating is removed and a fresh supply 

 of air is admitted, a new growth soon appears, and if a con- 

 stant supply of air is maintained, various molds may grow 

 until all the fruit is destroyed. 



225. Reproduction of bread mold. In addition to vegetative 

 reproduction by means of stolons, this mold also reproduces 

 itself both asexually and sexually. Upon the ends of upright 

 hyphse, sporangia are produced (fig. 184). In the develop- 

 ment of the sporangia, first a transverse wall cuts off a small 

 tip of the upright stalk. This tip cell grows rapidly until it 



