156 



FUNDAMENTALS OF CYTOLOGY 



further fact that the egg contains one plastid while the sperm contributes 

 none at syngam.y. 



Apospory and apogamy appear to be very rare in bryophytes in 

 nature. The ease with which apospory can be induced in mosses, how- 

 ever, renders these plants particularly valuable in the study of polyploidy. 

 By placing small pieces of immature sporophyte stalks under suitable 

 cultural conditions, they can be made to produce gametophytes by 

 regeneration. These gametophytes are diploid, and their diploid 

 gametes unite to form tetraploid sporophytes. This process can be 

 repeated, giving tetraploid gametophytes and octoploid sporophytes. 

 Sterility in some degree accompanies these new chromosomal states. 

 By means of hybridization, much higher degrees of polyploidy have been 

 obtained. Chromosome doubling can also be induced by chilling moss 

 protonemata and by injecting young spore capsules of liverworts with 

 certain chemicals. 



Fig. 114. — Behavior of plastids during sporogenesis in a moss {Polytrichum) . a, com- 

 pletion of plastid division in sporogenous cell, b, thready condition of plastids in sporocyte 

 about to divide, c, nucleus divided into four (three visible) ; immediately after cytokinesis 

 each spore will contain one nucleus and one plastid. d, young spore with two plastids 

 formed by division. The cells of the gametophyte have more plastids. {After T. E. 

 Weier.) 



Algae. — The variety of ways in which asexual reproduction by spores 

 and sexual reproduction by gametes are correlated in the life cycles of 

 algae is of great interest to both cytologists and students of phylogeny, 

 for it affords some basis for speculation as to the origin of the conditions 

 observed in other groups of organisms. In the paragraphs below we 

 shall therefore use the life cycle as a general basis of description, directing 

 attention here and there to special cytological features. 



Considering first the green algae, it is found that some of the most 

 familiar genera, such as Vlothrix, Oedogonium, and Spirogyra, have the 

 reduced chromosome number throughout the life cycle except in the 

 zygote. They are therefore termed haplonts and show no alternation of 

 vegetative generations. The Ulothrix plant reproduces asexually by 

 means of motile zoospores. Under appropriate conditions motile 

 biciliate gametes similar in form to the zoospores are produced, and 

 these unite two by two to form zygotes. The two which unite, although 

 morphologically alike, are "plus" and "minus" (female and male?) in 

 reaction and come from different filaments: the plants are heterothallic. 



