162 



PLANT BIOLOGY 



may not be still further subdivided. Hence, a fern leaf is usually 

 compound, and is strikingly graceful in its appearance. 



Beneath the ground the fronds grow from a horizontal stem called 

 the rhizome, which is more or less enlarged for food storage, depend- 

 ing on the kind of fern. To this rhi- 

 zome are attached the roots by which 

 the plant is supplied with soil-water. 

 The fern plant, therefore, like seed- 

 bearing plants, has all three kinds of 

 nutritive organs (roots, stem, and leaves), 

 and carries on carbohydrate manufacture 

 in the green fronds, storing away the 

 food in the rhizome, since the leaves die 

 to the ground each year. The follow- 

 ing spring the tiny leaves push up 

 through the ground from the under- 

 ground stem, unrolling and spreading 

 their leaflets from the base to the tip. 



176. Fern spores. On the under 

 surface of some of the leaflets of the 

 ferns named above are little dots which 

 are often brown. These are known as 

 fruit-dots (son). Each fruit dot, if ex- 

 amined with a microscope, is found to 

 consist of several smaller objects known 

 as spore-cases. (B, C, Fig. 82.) When 

 these tiny spore cases are ripe, they open, 

 often with considerable force, and eject a 

 powder, each particle of which is called 

 a spore. (Fig. 82, A.) Each spore con- 

 sists of a single cell. 



FIG. 83. Development of 

 fern plant. 



A, a germinating fern spore ; 

 B, a later stage in germina- 

 tion ; C, a full-grown pro- 

 thallus, showing rhizoids, 

 antheridia (or spermaries, 

 and archegonia (or 

 ovaries; Z>, section of 

 antheridia (or spermary) ; 

 E, a sperm cell ; F, a section 

 of archegonia (or ovary) 

 containing an egg-cell ; G, 

 young fern plant develop- 

 ing from a fertilized egg- 

 cell in an ovary, still at- 

 tached to the heart-shaped 

 prothallus. (Parker.) 



177. Fern prothallus. When the spores fall to the ground and 

 conditions are favorable, they start to germinate, and each finally 

 produces a small green, heart-shaped plant known as a prothallus 

 (Fig. 83, C). The prothallus, though tiny, consists of a great 



