52 MORPHOLOGICAL DEVELOPMENT. 



longer attachment will become an established characteristic; 

 and, by persistence in this process, permanent attachment 

 will result when permanent attachment is advantageous. 

 That disunion is really a consequence of relative innutrition, 

 and union a consequence of relative nutrition, is clear a 

 posteriori. On the one hand, the separation of the new indi- 

 viduals, whether in germs or as developed aggregates, is a 

 dissolving away of the connecting substance ; and this implies 

 that the connecting substance has ceased to perform its 

 function as a channel of nutriment. On the other hand, 

 where, as we see among Phamogams, there is about to take 

 place a separation of new individuals in the shape of germs, 

 at the point where the nutrition is the lowest, a sudden 

 increase of nutrition will cause the impending separation to 

 be arrested; and the fructifying elements, reverting towards 

 the ordinary form, thereupon develop in connexion with the 

 parent. Turning to the Archegoniates, we find among 



them many indications of this transition from discontinuous 

 development to continuous development. Thus the Liverworts 

 give origin to new plants by cells which they throw off from 

 their surfaces; as, indeed, we have seen that much higher 

 plants do. " According to Bischoff," says Schleiden, " both 

 the cells of the stem (Jungermannia [now Lophocolea] biden- 

 tata) and those of the leaves {J. exsecta) separate themselves 

 as propagative cells from the plant, and isolated cells shoot 

 out and develop while still connected with the parent plant 

 into small cellular bodies (Metzgeria furcata), which separate 

 from the plant, and grow into new plants, as in Mnium andro- 

 gynum among the Mosses." Now in the way above explained, 

 these propagative cells and proliferous buds, may continue 

 developing in connexion with the parent to various degrees 

 before separating; or the buds which are about to become 

 fructifying organs may similarly, under increased nutrition, 

 develop into young fronds. As Sir W. Hooker says of the 

 male fructification in Metzgeria furcata, — " It has the appear- 

 ance of being a young shoot or innovation (for in colour 



