330 



PLANT STUDIES 



"VT 



An ordinary fern sporangium consists of a slender stalk 

 and a bulbous top which is the spore case (Fig. 296, 6). 

 This case has a delicate wall formed of 

 a single layer of cells, and extending 

 around it from the stalk and nearly to 

 the stalk again, like a meridian line about 

 a globe, is a row of peculiar cells with 

 thick walls, forming a heavy ring, called 

 the anmdus. The annulus is like a bent 

 spring, and when the delicate wall be- 

 comes yielding the spring straightens 

 violently, the wall is torn, and in the re- 

 coil the spores are discharged with consid- 

 erable force (Fig. 301). This discharge 

 of fern spores may be seen by placing 

 some sporangia upon a moist slide, and 

 under a low power watching them as they 

 dry and burst. 



215. Heterospory. — This phenomenon 

 appears first among Pteridophytes, but it 

 is not characteristic of them, being en- 

 tirely absent from the true Ferns, which 

 far outnumber all other Pteridophytes. 

 Its chief interest lies in the fact that it 

 is universal among the Spermatophytes, 

 and that it represents the change which 

 leads to the appearance of that high 

 group. It is impossible to understand 

 the greatest group of plants, therefore, 

 without knowing something about heter- 

 ospory. As it begins in simple fashion 

 among Pteridophytes, and is probably 

 the greatest contribution they have made 

 to the evolution of the plant kingdom, 

 unless it be the leafy sporophyte, it is best explained 

 here. 



Fig. 300. A moonwort 

 {Botrychium), show- 

 ing the leaf dififerenti- 

 ated into foliage and 

 sporophyll branches. 

 —After Strasbur- 



