258 LECTUEES ON BIOLOGY 



higher multicellular organisms, but must in a short time lead 

 to degeneration ; on that point opinions are greatly divided. But if 

 we consider that the pyramid-poplar and numerous other ancient 

 domesticated plants, such as the vine, fig-tree, date-palm, banana- 

 tree and many other useful plants, have propagated their species 

 since times immemorial by an exclusively asexual method, (i.e., by 

 layers), without having suffered the least decrease in strength 

 or the quality of their fruits, it would seem that a natural limit 

 does not exist and that the asexual propagation can in many 

 cases replace sexual reproduction. But the pyramid-poplar is 

 frequently mentioned as a case against this hypothesis, because 

 it has lately everywhere commenced to sicken and die out. The 

 cause of this phenomenon is, however, probably not so much to 

 be found in their mode of reproduction as in an undue increase in 

 parasites and other unfavourable external conditions, against 

 which the seed would be equally defenceless. 



As a kind of inner gemmation we may describe a method of 

 reproduction which has hitherto been observed with certainty 

 only in fresh-water sponges. At the beginning of the inclement 

 season there are formed in the interior of the sponge-body globular 

 accumulations of cells which all originate from the central ger- 

 minal layer. The external cells form themselves into a mantle 

 consisting of one cell-layer, and secrete a solid cover which is 

 supported by numerous silica-needles, the amphidiscs. When 

 severe frost sets in the sponge-body perishes, but these gemmulae 

 are, thanks to their power of resistance, able to live through the 

 winter. They sink to the bottom of the water and remain there 

 apparently lifeless, but when spring returns the encysted sponge- 

 particles leave their protecting membrane, commence to grow, 

 and proceed to reproduction. 



In the moss animalcules found in fresh water we observe 

 similar multicellular interior buds, called statoblasts, which after 

 a long period of rest may develop into a new colony of Bryozoa. 

 The principal task of these buds is to help the animalcules 

 through the perilous times of drought and frost, and later, when 

 the warmer weather has returned, to bring about rapid multipli- 

 cation of the species. In order to be able to discharge these 

 functions the statoblasts are enclosed in a hard chitinous shell 

 which effectually protects the young germ against all the dangers 



