352 



MALE FERN. 



anterior end, in which lies the growing-point; the thickened 

 median portion, or cushion, and the thin lateral expansion 

 or wing on either side ; the rhizoids, springing from the 

 underside, chiefly from the cushion. Note that the pro- 

 thallus is not usually pressed quite closely to the soil, but 

 is tilted upwards in front, so that under moist conditions 

 there is a water-holding space between the underside of the 

 prothallus and the soil ; if the prothalli are crowded, they 

 slope steeply upwards in front, especially if grown in a 

 rather deep vessel. 



495. Prothallus with Sexual Organs (Fig. 87). 

 Mount a number of prothalli in water, with the lower 

 surface uppermost, and examine with the microscope. 



Fig. 87. FERN. A Prothallus, seen from lower side. 



Note the general features, and the following additional 

 points : 



(1) The polygonal thin-walled cells of the lateral wings, 

 each containing numerous chloroplasts in addition to pro- 

 toplasm, nucleus, and cell-sap. 



(2) The several-layered cushion, consisting of similar 

 cells those on the underside of the cushion usually contain 

 relatively few chloroplasts, and some have grown out to 

 form 



