126 Elementary Biology. 



may by subsequent division become a double layer, to which 

 the name of tapetum is given. The remainder of the 

 archesporium gives rise by subdivision to what are known 

 as the mother spore-cells, usually sixteen in number. The 

 tapetal cells do not remain long in existence as distinct cells, 

 but deliquesce and form a sort of watery jelly in which the 

 sixteen mother spore-cells float. Thereafter each mother 

 spore-cell divides into four spores, each of which in the 

 process of growth assumes the characters described above 

 as possessed by the adult spore. 



The various changes which take place in the develop- 

 ment of a sporangium must be carefully borne in mind when 

 the mode of origin of the sporangia of the higher plants 

 comes to be considered. The main features in their develop- 

 ment will be found to be the same, and homologies at present 

 obscured by an old-fashioned terminology will, in the light 

 of the life-history of the fern, become at once evident. 



We have now considered at sufficient length the morpho- 

 logy of the vegetative and asexual reproductive organs of 

 Pteris ; we must now follow the spore through the changes 

 which it undergoes when sown on moist, warm soil. 



The first appearance of the future plant is the protrusion 

 of a minute protoplasmic bud through the exosporium. 

 This bud by growth and division becomes an elongated 

 thread, which by future division becomes a flat expansion, 

 the cells of which contain chlorophyll. Growth takes place 

 chiefly at the distal end of the thallus, and in such a manner 

 that the thallus becomes heart-shaped. Though at first 

 composed of only one layer of cells, two or more layers soon 

 make their appearance. The thallus is especially thick at 

 some distance behind the growing point, and rises in a 

 mound-like elevation on the under surface. From this 

 mound root-hairs, or rhizoids, are given off which penetrate 

 the soil and perform the same function that true root-hairs 

 perform in the spore-producing plant. From the same 

 mound are developed the female sexual organs, whilst the 



