FERNS. 259 



dot, when it disappears in a twinkling. The water has been taken 

 in under such pressure that it has absorbed all the gas, and the 

 farther pressure in most cases closes the partly opened sporangium 

 more completel)'. 



539. Now we should add glycerine again and draw out the 

 water, watching the sporangia at the same time. We see that 

 the sporangia which have opened and snapped once will do it 

 again. And so they may be made to go through this operation 

 several times in succession. We should now note carefully the 

 annulus, that is after the sporangia have opened by the use of 

 glycerine. So soon as they have snapped in the glycerine we can 

 see those minute spheres of gas again, and since there was no air 

 on the outside of the sporangia, but only glycerine, this gas must, 

 it is reasoned, have been given up by the water before it was all 

 drawn out of the cells. 



540. The common polypody. — We may now take up a few other ferns for 

 study. Another common fern is the polypody, one or more species of which 

 have a very wide distribution. The stem of this fern is also not usually seen, 

 but is covered with the leaves, except in the case of those species which grow 

 on the surface of rocks. The stem is slender and prostrate, and is covered 

 with numerous brown scales. The leaves are pinnate in this fern also, but we 

 find no difference between the fertile and sterile leaves (except in some rare 

 cases). The fruit-dots occupy much the same positions on the under side of the 

 leaf that they do in the Christmas fern, but we cannot find any indusium. In 

 the place of an indusium are club-shaped hairs as shown in fig. 291. The en- 

 larged ends of these clubs reaching beyond the sporangia give some protection 

 to them when they are young. 



541. Other ferns. — We might examine a series of ferns to see how different 

 they are in respect to the position which the fruit dots occupy on the leaf. The 

 common brake, which sometimes covers extensive areas and becomes a trouble- 

 some weed, has a stout and smooth underground stem (rhizome) which is often 

 12 to 20 C7n beneath the surface of the soil. There is a long leaf stalk, which 

 bears the lamina, the latter being several times pinnate. The margins of the 

 fertile pinns are inrolled, and the sporangia are found protected underneath 

 in this long sorus along the margin of the pinna. The beautiful maidenhair fern 

 and its relatives have obovate pinnae, and the sori are situated in the same posi- 

 tions as in the brake. In other ferns, as the walking fern, the sori are borne 

 along by the side of the veins of the leaf. 



542. Opening of the leaves of ferns. — The leaves of ferns open in a peculiar 

 manner. The tip of the leaf is the last portion developed, and the growing 



