174 ORGANISMS ILLUSTRATING RIOLOGICAL PRINCIPLES 



Great numbers of tiny spores are produced by division of the dense 

 terminal portions of the sporangiophores. As a sporangium becomes 

 mature an outer wall is formed and the spores turn black in color. 



When this outer wall 

 breaks, the minute spores 

 are scattered far and wide 

 by air currents. 



Molds also reproduce 

 sexually, by means of con- 

 jugation. Rhizopus has 

 two different strains of 

 mycelia, one of which is 

 called a plus ( + ) and the 

 other a minus ( — ) strain. 

 If hyphae of two such 

 strains come in contact 

 with each other, zygo- 

 spores are formed. Short, 

 club-shaped branches are 

 developed from the hy- 

 phae, the dense proto- 

 plasmic tips are cut off from the end of each by cell walls, and these 

 "cells," each of which contains several nuclei, unite to form a 

 zygote. The zygote with the hyphae which develop from it proba- 

 bly represents the diploid stage of chromosome in the life cycle, 

 the haploid stage being reached when the spores on the sporangium 

 germinate. 



The fungi are of even more interest by reason of their method of 

 nutrition. They are typically neither holozoic nor holophytic, since 

 they live as saprophytes on dead organic materials. This means that 

 they must absorb food materials which are supplied to them from 

 outside sources after digesting them by means of enzymes, when 

 absorption takes place through the plasma membrane of the cell. 



Alternation of Generations in the Plant Kingdom 



The most important difference in the life cycle between the Bryo- 

 phytes or Mosses and lower forms, aside from a greater differentiation 

 of the plant body, is the alternation of an asexual with that of a sexual 

 generation in the hfe cycle. The asexual generation, which produces 

 spores, is called the sporophyte, while the sexual generation, which 



Reproduction in bread mold (Rhizopus nigri- 

 cans). Read the text and then explain the 

 diagrEun. 



