ii4 THE POLYSPORANGIATE STATE 



the total output of spores from the numerous sporangia may be virtually 

 unlimited : moreover, their production may be extended over many years 

 on the same individual plant. Since, in the homosporous forms, each 

 single spore is small, and therefore conveys with it only a small store 

 of nutriment, the risks on germination are great ; a reasonable chance of 

 success is, however, secured by their large number. 



But with the heterosporous condition complications arose. Owing to 

 the storage arrangements in the enlarged female spore, this more specialised 

 state leads to economy in number of the spores necessary to secure survival 

 and spread of area ; for each female spore carries with it, in its higher 

 store of nourishment, a higher probability of successful establishment of 

 an embryo, and a sufficient degree of propagative probability can thus 

 be attained with a moderate number of spores. Hence heterosporous 

 types may be expected to present examples of reduction of number, not 

 only of sporogenous cells, but also of sporangia. That is seen to be actually 

 the case, and it might be illustrated by numerous examples. It will then 

 be in homosporous types, which are certainly the more primitive, that 

 we shall expect to meet with the best evidence as to the origin of the 

 polysporangiate state, or with traces of increase in number of sporangia ; 

 in fact, they will illustrate more faithfully than heterosporous forms the 

 upgrade of complexity of their spore-producing parts. 



On grounds of nutrition of the spores, and of stability at the critical 

 stage when the spore-mother-cells are floating in fluid, there is a clear 

 advantage in the segregation of the spores into separate pockets the 

 sporangia as against any method of indefinite enlargement of a single 

 sac. It is probably such conditions as these which have also determined 

 the limits of size of the individual sporangia of the Pteridophytes, and led 

 to some degree of uniformity in their dimensions. But still considerable 

 variations in size of the sporangia are found to occur, even in close 

 juxtaposition on the same plant : this is most conspicuous in the 

 Eusporangiate forms. Sometimes the difference in size seems to be 

 dependent on nutrition ; for instance, it is usual to find about the upper 

 and lower limits of the fertile strobilus of Lycopods sporangia of smaller 

 size than those about the middle of the fertile region : the same is the 

 case in the Psilotaceae and in Equisetum. But in other cases this simple 

 explanation will not suffice, for smaller sporangia may be found dis- 

 tributed between the larger ones : this is especially so in the sori of 

 the Marattiaceae, and a general survey shows that in many of the 

 Eusporangiate forms the single sporangium is not quantitatively a definite 

 unit. 



But though there may thus be wide variation of size of the individual 

 sporangia in certain Pteridophytes, still in others their dimensions are 

 often very uniform. In the Leptosporangiate Ferns, indeed, the number 

 of spores in a single sporangium is often strictly constant. In that case 

 change in the output of spores on the plant is effected by change in the 



