348 



MOSSES AND FERNS 



CHAP. 



the growth of the prothallium is exactly opposite to that of 

 the first rhizoid (bi-polar germination), and Kny ((5), p. 12) 

 lays a good deal of stress upon this, as distinguishing Osmunda 

 from the Polypodiacese ; but it is not at all uncommon for O. 

 Claytoniana, especially, to have the axis of growth of the rhi- 

 zoid almost or quite at right angles to that of the prothallium, 

 exactly as in the Polypodiacese. Where the germination is 

 truly bi-polar the exospore is pushed up with the growing pro- 

 thallium, and appears like a cap at its apex, but if the rhizoid is 

 lateral, the exospore remains at the base. 



In 0. Claytoniana there are usually several transverse walls 

 A. 



B. 



FIG. 192. Osmunda cinnamomea. A, Young prothallia; B, an older prothallium, X26o. 



formed before any longitudinal ones, but in O. cinnamomea 

 and O. regalis it is quite common to have the first transverse 

 wall followed by a longitudinal wall in each cell, so that the 

 four primary cells are arranged quadrant-wise (Fig. 192, A, 

 c). Rarely the first wall in the prothallial cell is longitudinal, 

 as is often the case in Equlsetum, and sometimes the first divi- 

 sions are in three planes, so that a cell mass is formed at once, 

 as so often occurs in the Marattiacese. Where a filamentous 

 protonema is formed, a two-sided apical cell is soon established 

 in exactly the same way as in Onoclea. Where the four quad- 

 rant cells are formed, one of the terminal ones becomes at once 

 the apical cell. 



