REPRODUCTION IN PLANTS 



255 



high and low, this direct vegetative process forms an important or even 

 the chief method of reproduction. 



REPRODUCTION IN THE THALLOPHYTES 



We have seen that the thallophytes include the algae, fungi, and bac- 

 teria, and form the lowest of the four great groups of the plant kingdom. 

 Among the Thallophyta we find not 

 only the most primitive of all types 

 of reproduction but also encounter 

 a graded series of reproductive 

 processes, some of which appear to 

 illustrate the origins of sex and the 

 beginnings of the alternation of 

 generations. 



Reproduction by simple fission. 

 In the blue-green algae, bacteria, 

 and some of the most primitive 

 fungi, the protoplasm of the cells 

 shows little or no differentiation. 

 There is no division into nucleus 

 and cytoplasm. Here reproduction 

 is by the simplest type of fission 

 and does not even involve mitotic 

 processes. In some of the lowest of 

 the green algae, possessed of a 

 nucleus and various cytoplasmic 

 structures, all reproduction is still 

 by fission, but by a type of fission 

 that now involves mitosis. 



Budding is a variant type of 

 fission illustrated by the reproduc- 

 tion of yeasts. It differs from typical 

 fission in that the parent cell 

 produces a small bud which grows 

 gradually to full size and separates 

 without the parent cell losing its 

 identity. But it is like fission in 

 that the yeast cell is able to produce a daughter cell without a stimulus 

 from another individual. 



Sexual reproduction in the thallophytes. In most of the green algae 

 fission is not the only mode of reproduction, though it remains the chief 

 method for rapid multiplication during the growing season. From time to 

 time, however, sexual reproduction occurs, usually in response to unfavor- 



Fig. 17.1. Sexual reproduction in the fila- 

 mentous green alga Spirogyra. The struc- 

 ture of the vegetative cells is shown in A, 

 with the characteristic spiral chloroplasts 

 and a central nucleus. At B two cells in 

 adjacent filaments are preparing to conju- 

 gate. At C the conjugation tube has been 

 completed and the protoplasm of one of the 

 two cells (corresponding to a male gamete) 

 is passing through the tube. At D conjuga- 

 tion has been completed, a zygote has been 

 formed, and a resistant wall has been se- 

 creted around the zygote to form a zygo- 

 spore. The two conjugating cells in Spiro- 

 gyra are isogametes. (Modified from Turtox 

 chart, courtesy General Biological Supply 

 House, Inc.) 



