

396 PROVISIONAL HYPOTHESIS Chap. XXVII. 



Another difficulty may be here noticed ; we have seen that 

 there is an important difference in the frequency, though not 

 in the nature, of the variations in plants propagated by sexual 

 and asexual generation. As far as variability depends on the 

 imperfect action of the reproductive organs under changed con- 

 ditions, we can at once see why seedlings should be far more 

 variable than plants propagated by buds. We know that 

 extremely slight causes, — for instance, whether a tree has been 

 grafted or grows on its own stock, the position of the seeds 

 within the capsule, and of the flowers on the spike, — sometimes 

 suffice to determine the variation of a plant, when raised from 

 seed. Now, it is probable, as explained when discussing alter- 

 nate generation, that a bud is formed of a portion of already 

 differentiated tissue; consequently an organism thus formed 

 does not pass through the earlier phases of development, and 

 cannot be so freely exposed, at the age when its structure 

 would be most readily modified, to the various causes inducing 

 variability ; but it is very doubtful whether this is a sufficient 

 explanation of the difficulty. 



With respect to the tendency to reversion, there is a similar 

 difference between plants propagated from buds and seed. 

 Many varieties, whether originally produced from seed or buds, 

 can be securely propagated by buds, but generally or invariably 

 revert by seed. So, also, hybridised plants can be multiplied to 

 any extent by buds, but are continually liable to reversion by 

 seed, — that is, to the loss of their hybrid or intermediate cha- 

 racter. I can offer no satisfactory explanation of this fact. Here 

 is a still more perplexing case : certain plants with variegated 

 leaves, phloxes with striped flowers, barberries with seedless fruit, 



all be securely propagated by the buds on cuttings ; but the 



buds developed from the roots of these cuttings almost invariably 

 lose their character and revert to their former condition. 



Finally, we can see on the hypothesis of pangenesis that 

 variability depends on at least two distinct groups of causes. 

 Firstly, on the deficiency, superabundance, fusion, and trans- 

 position of gemmules, and on the redevelopment of those 

 which have long been dormant. In these cases the gemmules 

 themselves have undergone no modification ; but the mutations 

 in the above respects will amply account for much fluctuating 











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