666 TEXTBOOK OF BOTANY 



As the flat expanse of cells forming the fern gametophyte grows, 

 rhizoids appear on the lower side; and soon afterward antheridia de- 

 velop near them (Fig. 312). The antheridia are comparatively simple 

 structures, with an external layer of several cells enclosing the sperm 

 mother cells, in each of which one sperm develops. The sperms have a 

 spirallv twisted bodv with 40 or 50 long cilia. The archegonia develop 

 as the gametophyte matures, and are also located on the under side 

 near the notch or youngest part of the thallus. 



Fertilization. The sperms are released by the swelling and bursting of 

 the mature antheridium when water collects under the gametophyte. 

 Under similar conditions the archegonium opens, and the products of 

 the disintegration of the neck canal cells diffuse into the water. The 

 sperms swim toward the regions of greatest concentration of these 

 diffusing substances, and one of the spenns after entering the arche- 

 gonium fuses with the egg cell, forming a zygote. When fertilization 

 has taken place in one or two of the archegonia, the further develop- 

 ment of the remaining archegonia of the gametophyte ceases. Conse- 

 quently one sporophyte ordinarily develops from a zygote in each fern 

 gametophyte, rarely two of them. If the young embryos of the sporo- 

 phytes are removed as rapidly as they begin to develop, the feni game- 

 tophytes continue to grow vegetatively for years. Apparently a hormone- 

 like substance formed in the young sporophyte inhibits not only the 

 further development of other embryos, but also the further growth of 

 the prothallus, or gametophyte. 



Embryo of the sporophyte. The zvgote germinates directly after fer- 

 tilization. Cell division soon results in an embryo with four distinct 

 regions : ( 1 ) the foot, or holdfast, by which the embryo is attached for 

 a short time to the gametophyte; (2) a root tip, which rapidly elongates 

 and penetrates the soil; (3) a leaf primordium, from which the first leaf 

 of the sporophyte develops; and (4) a stem tip, which elongates slowly, 

 and from which successive leaves and adventitious roots develop. The 

 sporophvte is thus at first dependent on the gametophyte, but it soon 

 becomes green and develops rapidly; and the thallus disintegrates. From 

 the embryo the mature sporophvte grows. 



Chromosome numbers. The cells of the sporophyte all have the dip- 

 loid number of chromosomes. Reduction division occurs in the spore 

 mother cells during the formation of spores in the sporangia on the 

 leaves. These spores, the cells of the subsequent gametophytes, and the 

 sperms and eggs have the monoploid number of chromosomes. 



