POLLINATION. 397 



results if pollinated under confinement, because conditions are 

 under control. 



Crossing of Flowerless Plants. Ferns, lycopodiums, and 

 their allies, pass / ^- = ==~^m^~ 



through two stages / 



of development, and HIIHBHHHHHHMUllllllllilllllilllllM 

 fertilization takes 

 place only in the 

 first stage. When 

 spores germinate, a 

 small, thin, green . Fig. 105. Pollinating Kit, closed, 

 tissue spreads over 



the soil. This tissue is the prothallus (or prothallium). Upon 

 the prothallus, or somewhat sunken in it, the sexual organs 

 appear. They are minute aggregations of cells. Some of these 

 aggregations develop into sperm or male organs and some into 

 germ or female organs. The sperm organ is known as an anther- 

 idium and the germ organ as an archegonium. Spermatozoids 

 are formed in the antheridium, and these enter the archegonium 

 and fertilize the germ cell. This fertilized germ cell then devel- 

 ops into the second stage of the species, or into that part which 

 we know as the fern or the lycopod. During this second stage, 

 the plant bears leaf -like organs and it also produces numerous 

 spores. These spores will produce the prothallus again when 

 sown. Ferns, therefore, are fertilized but once during their 

 lifetime, and the spores are not the direct result of fertilization 

 as are the seeds of flowering plants. 



If ferns and other flowerless plants are to be crossed, there- 

 fore, the operation must be performed in the prothallic stage. 

 It was long a matter of doubt among botanists as to whether 

 crossing is possible among these plants, but it is now known that 

 it does occur. It has been brought about repeatedly in cultiva- 

 tion. The sperm bodies are not transferred by hand, but the 

 spores of the species between which crosses are desired are 

 sown together and the transfer is allowed to take place naturally. 

 The prothallia of ferns are nearly always dioecious (sexes borne 

 on different plants), so that crossing in such cases is not im- 



