42 



PLANT MORPHOLOGY 



separate plants unite to form a zygote. Instead of becoming a thick- 

 walled resting cell, the zygote germinates immediately and produces a 

 plant that bears only zoospores. 



Thus Ulva illustrates the phenomenon of alternation of generations. 

 Two separate plants, one producing gametes and the other spores, are 

 involved in each life cycle and, although they look alike, the gamete- 

 producing plants are haploid and the spore-producing plants are diploid. 



Fig. 30. Ulva lactuca, the sea lettuce, about one-half natural size. The bright green 

 thallus is only two layers of cells thick. (After Thuret.) 



The doubling of chromosomes, resulting from the conjugation of two 

 gametes, is carried over by the zygote to the cells of the spore-producing 

 plant. The reduction of chromosomes takes place when the zoospores 

 are produced. These haploid spores give rise to the gamete-producing 

 plants. The haploid plants are called gametophytes and the diploid 

 plants sporophytes. Because the two kinds of plants are alike vegeta- 

 tively, Ulva displays an isomorphic alternation of generations. 



Coleochaete. Coleochaete is a small fresh-water alga that usually 

 grows attached to leaves and stems of aquatic seed plants, such as water 



