198 



PRACTICAL BOTANY. 





FIG. 124. A spore 

 of Pteris serrulata. 

 (After Suminski.) 



arranged to form a linear aggregate ; this arrangement is 



succeeded by a superficial aggregate, and this by a solid 

 aggregate. In passing through these 

 stages the protonema is gradually trans- 

 formed into the prothallium, which is 

 the gametophyte form of the plant. 

 (See Figs. 124, 125, 126, 127, 110.) 



11. If the cultivation of Number 10 

 has not produced prothallia in from six 



to ten weeks, search for them on the Fern pots, the ground, 



and walls of greenhouses where Ferns are cultivated. 



They are very small one eighth to one third of an inch 



in diameter. 



Under low power draw the prothallium as seen from 



above, and again as seen from the under side. Notice the 



distinct heart shape. Does this recall the dichotomous 



division of Liverworts ? Do you find rhizoids ? If so, 



where are they situated ? Why are they not true roots ? 



Under high power examine some of the cells near the 



margin of the prothallium. Do they contain chromato- 



phores ? Nuclei ? Starch ? 



In testing for starch use pro- A 



thallia that have been kept 



in alcohol. 



With the high power search 



the region just below the 



sinus of the heart on the 



under side of the prothallium 



for archegonia. (See Fig. 



110.) They are borne on a 



thickened region of meriste- 



matic tissue called the cushion. 



What is their shape ? Of 



how many cells does each consist ? Can you find the 



canal or opening into the archegonium ? Draw. 



FIG. 125. A, a spore of Pteris 

 serrulata after it has begun to 

 germinate; B, the same, farther 

 advanced ; S, the spore ; r, the. 

 rhizoid ; p, the young protonema. 

 (After Suminski.) 



