THE FORMATION OF GERM-CELLS 211 



been given, and is as follows : — in addition to their active meso- 

 derm-idioplasm, the cells which will give rise to germ-cells carry 

 along with them a certain amount of germ-plasm in an unalterable 

 condition. De Vries, and those botanists who agree with him, 

 consider my answer superfluous. Every one. of course, is at 

 liberty to reject the solution of a problem, but in that case he 

 must not claim to have explained it. 



I now come to the consideration of de Vries's ' accessory 

 germ-tracks.'' As has already been stated, this term is used to 

 describe those series of cells which give rise to germ-cells 

 through the agency of 'adventitious buds.'' According to 

 Sachs,* adventitious buds correspond to those growing points 

 which are not derived from those already present, or ' from 

 embryonic tissue already present,' but which ' originate at places 

 where the tissue has already passed over into the permanent 

 condition, — in fully-developed roots, in the interfoliar parts of 

 shoot-axes, and more particularly in foliage leaves, the tissues of 

 which are already completely differentiated and developed.'' 



In my former essays I have endeavoured to account for these 

 ' adventitious ' buds, — such as are formed, for instance, in a 

 Begonia leaf when it is placed on damp soil, — by supposing them 

 to be adaptations of particular species of plants to this peculiar 

 method of reproduction : I assumed that certain series of cells 

 which in these species take part in forming the leaves contain 

 unalterable and inactive germ-plasm in addition to their own 

 active idioplasm. 



In opposition to this interpretation much may be, and in fact 

 has already been, said, and the principal objections must now 

 be considered. 



It has, in the first instance, been urged that the capacity pos- 

 sessed by leaves, roots, and so on, for producing adventitious buds, 

 cannot be regarded as an adaptation, because so many cases 

 are known in which this process only occurs exceptionally, and is 

 of no advantage to the plant. There can be no doubt, however, 

 that the power possessed by Begonia, Bryophylluni, Car da mine 

 pratensis, and Nasturtmm officinale, of giving rise to buds in 

 those parts in which they are not formed in most plants, is due 

 to an arrangement peculiar to these plants. Neither in Begonia 

 nor in Bryophyllmn can the buds and young plants arise from 



* ' Lectures on the Physiology of Plants," p. 477. 



