PEKNS. 133 



spore itself elongates at the same time and becomes sep- 

 tate, the septa at first arising at right angles to its 

 direction of growth. By further growth, and a series of 

 divisions in different directions, the mature prothallium 

 is finally produced. While the prothallium is in the 

 early, or filamentous stage of its development, the form 

 and contents of its cells and other structural details are . 

 easily observed. Full descriptions, accompanied by care- 

 ful drawings, should be made.^ 



II. The mature prothallium may be raised successfully 

 by taking care of the specimens that have been started 

 as directed above; but since they require weeks, or even 

 months, to attain their full development, it is more con- 

 venient to obtain prothallia from conservatories where 

 ferns are cultivated. In the pots containing ferns, or on 

 the surface of the moist earth near by, one can frequently 

 find excellent specimens. They are generally heart-shaped, 

 a few millimeters to a centimeter in diameter, of a delicate 

 green color, and so much like small liverworts as some- 

 times to deceive experienced collectors. 



An uninjured specimen that has been carefully washed, so 

 as to remove the adherent particles of earth, shows under 

 the microscope a deep anterior depression, sinus, and back 

 of this a thickened portion of the prothallium, sometimes 

 called the cushion. The latter is several layers of cells in 

 thickness, while the parts nearer the margin are but one 

 layer thick. Ehizoids in great numbers arise from the 

 lower surface. The growing point is at the base of the 

 depression. The arrangement of the cells at this point 

 indicates their order of development, which is readily 



1 For a model of. Campbell, Development of the Ostrich Fern. 

 Memoirs, Boston Soo. Nat. Hist., Vol. IV, No. II (1887). 



