EXERCISE 123 



In all except the lowest families of plants there are two modes of reproduction and two distinct 

 generations, each reproducing in its own way. One generation, reproducing by means of sperms and 

 eggs, is called the sexual generation ; the other, reproducing by means of spores, is called the sexless, or 

 asexual, generation. 



Problem. What is the life history of a plant having alternation of sexual and asexual generations ? 



What to use. Fresh or dried moss of some large variety, with the "fruit" stalks, complete; some 

 without the fruits ; microscope with slides etc. ; magnifying glass ; section cutter ; prepared micro- 

 scopic slides. 



What to do. 1. If dry material is used, it should be - soaked up in water. Examine the plant with- 

 out fruit and note its main parts and the arrangement and character of the " leaves." This is the 

 sexual generation. Note the rosette of leaves at the top. Examine several specimens with the glass 

 to find structural differences. Make longitudinal sections through the tips of several specimens and 

 examine with the microscope. Compare your sections with the prepared ones. 



NOTE i. It is possible to find, among the hairlike growths within some of the rosettes, flask-shaped organs, the arche- 

 gonia, each containing a large egg cell in the basal portion ; and in other rosettes, club-shaped structures,- the antheridia, 

 each of which produces a large number of sperm cells. 



2. Examine some of the fruiting specimens and note the stalk and the spore capsule on top, the 

 hood over the capsule, the lid, and the structure of the mouth after the lid is removed. This is the 

 asexual generation. Study with the magnifying glass all the structures named. Make cross and 

 longitudinal sections of the spore case and study with the microscope. Compare your sections with the 

 prepared ones. 



NOTE 2. Sperm cells swim about by means of cilia, and some find their way down the neck of an archegonium, where 

 one unites with the egg cell. The fertilized egg cell begins at once to develop into a spore-bearing individual, absorbing 

 practically all its food from the parent plant the female sexual individual. The hood on top of the spore case is the 

 upper part of the archegonium, which continues to grow for a while after fertilization takes place. 



NOTE 3. The spore-bearing (asexual) individual is called the sporophyte. The gamete-bearing (sexual) individuals, 

 bearing the sperm or eggs, are called the gametophytes. 



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