202 



PLANT BIOLOGY 



objects when examined under the microscope (X about 200).. 

 They are spherical, green bodies, each surrounded by two spiral 

 bands attached to the spore at their intersection, s. These bands 

 exhibit hygroscopic movements by means of which the spores be- 

 come entangled, and are held together. This is of advantage to the 

 plant, as we shall see. All the spores are alike, but some of the//v>- 

 thallia grow to a greater size than the others. The large prothallia 

 produce only archegonia while the smaller ones produce antheridia. 

 Both of these organs are much like those of the ferns, and fertili- 



FlG. 299. — Equisetum arvense. 

 st, sterile shoot; f, fertile shoot showing the spike at a ; 6, sporophyll, with sporangia; 



zation is accomplished in the same way. Since the prothallia are 

 usually dioecious, the special advantage of the spiral bands, holding 

 the spores together so that both kinds of prothallia may be in 

 close proximity, will be easily understood. As in the fern, the 

 fertilized egg-cell develops into an equisetum plant. 



The sterile shoots (st, Fig. 299) appear much later in the season. 

 They give rise to repeated whorls of angular or furrowed branches. 

 The leaves are very much reduced scales, situated at the inter- 

 nodes. The stems are provided with chlorophyll and act as 

 assimilating tissue, nourishing the rhizome and the fertile shoots. 

 Nutriment is also stored in special tubers developed on the rhi- 

 zome. 



