418 PLANT MORPHOLOGY 



vegetative cells its ability to divide, but does so only after being liberated. 

 It differs from other cells not in power but in opportunity. Asexual 

 reproduction by means of spores is a feature of nearly all the green algae 

 and of most fungi, occurring even in some of the unicellular forms. 



It should be understood that the formation of vegetative spores never 

 involves a reduction of chromosomes. In fact, with few exceptions, they 

 are borne on a haploid plant body. Vegetative spores always give rise to 

 the same kind of plant body as the one that produced them. Spores 

 formed by meiosis from a diploid cell, like tetraspores in the red algae, 

 ascospores and basidiospores in the fungi, and all spores in the higher 

 plants, are not vegetative spores but meiospores and belong to the sexual 

 life cycle. 



In plants above the thallophyte level, asexual or vegetative repro- 

 duction is carried on by various means, such as gemmae, bulbils, bulbs, 

 tubers, runners, isolated branches, etc. Each of these consists of a group 

 or mass of vegetative cells isolated from the parent and capable of 

 reproducing it. 



Origin of Sex. Most algae producing vegetative zoospores, whether 

 unicellular or multicellular, also bear gametes on the same kind of vegeta- 

 tive body. Gametes not only resemble zoospores structurally but, in 

 isogamous forms, commonly intergrade with them. This indicates that 

 gametes have been derived from zoospores. It is not known what 

 induced reproductive cells to first unite in pairs, but the tendency soon 

 became a fixed habit. 



The fact that the zygote becomes a thick-walled resting cell in most 

 green algae suggests that the original function of sexual reproduction was 

 protection over a period of unfavorable conditions. In fact, experiments 

 have shown that the advent of such conditions induces gamete formation. 

 When conditions for vegetative growth are at their best, reproduction 

 does not occur. When they become somewhat less favorable, vegetative 

 activity begins to wane and spores are formed. As conditions become 

 more severe and the plant approaches the end of its growing season, 

 gametes appear. The conditions favoring gamete production in the green 

 algae are those that inhibit germination of the zygote and result in its 

 dormancy. When it germinates at the beginning of the next season, con- 

 ditions are usually not conducive to maximum vegetative activity. It is 

 possible that the formation of spores by the zygote, a feature of so many 

 green algae, is merely a response to these conditions. 



Differentiation of Gametes. In isogamy all the gametes are alike in 

 appearance and behavior. The fact that they pair, however, implies a 

 mutual attraction and indicates that a difference exists between them. 

 Each of the pairing gametes must represent an opposite sex. In heterog- 



