XIII] 



HETEROSPORY 



267 



msp 



It thus appears that notwithstanding the great variations of sporangial out- 

 put, the result per sorus is approximately uniform for the cases quoted from 

 a very natural family of Ferns. This table illustrates a principle of very wide 

 application in Ferns, viz. that diminution of productivity of the individual 

 sporangium does not necessarily lower the productivity of the sorus : for it 

 is often compensated by the larger number of the sporangia. Moreover, their 

 appearance is liable in gradate and mixed sori to be spread over a long 

 period of time, with advantageous results as regards nutrition. 



HETEROSPORY 



The essential feature of Heterospory as seen in Ferns is that, while the 

 spores which on germination produce antheridia and spermatozoids (micro- 

 spores) remain of the approximate size and number of the homosporous 

 spores, others which on germination produce one or more archegonia or ova, 

 grow to a relatively large size, but are fewer in number (megaspores). The 

 two types of spore are produced in different sporangia. Nevertheless the 

 modeof development of those spo- ^ 

 rangia remains the same as in re- 

 lated homosporous plants, of which 

 they may be held to be specialised 

 examples. The changes involved 

 in heterospory are thus intra- 

 sporangial, and do not appear to ttisI 

 have brought far-reaching effects 

 upon other parts than the contents 

 of those sporangia. 



In the vast majority of Ferns 

 the spores are all alike. Some 

 few exceptions occur of which the 

 most remarkable are those oi Pla- 

 tyzoma and Notholaena already 

 noted. Cultivation must decide 

 whether or not the differences 

 there seen are merely results of 



varying nutrition, or are initial Fig. 262.MicrosporangiaoftheSalviniaceae.^=^si7/- 

 . , 1 1 , la: msl.= the massulae; s^. = the stalk consisting 

 Steps towards a heterosporous sex- ^^ ^^^^ ^^^^ ^f ^^1,^ B^Sa/z^tma: .«./.=micro 



difference. This state is charac- spores; 5/. = stalk. (After Strasburger, from Engler 

 . . r 1 n/r M- 1 andPrantl.) ( x 80.) 



teristic of the Marsihaceae and 



Salviniaceae, families which differ so widely that it seems inevitable that 

 their heterospory must have been separately acquired. The most highly 

 specialised example is that of Azolla, which may serve as an example though 

 an extreme one. Both types of sporangia here arise by the growth and 



