1 42 THE CAUSES OF SPECIFIC SHAPE 



Asomatophytes as bacteria and yeast under special cultural conditions, and 

 by bryophytes and many thallophyta during the normal development 

 from the spore. Similarly in flowering plants all cells capable of producing 

 rooting buds must retain the general embryonic properties required for the 

 production of a new individual. Possibly in some cases such cells may 

 in part be adult, but may revert to the embryonic condition by a retro- 

 gressive change excited by the renewed growth. In Asomatophytes also, 

 an artificially induced modification of the germ-plasma may be eliminated 

 in the course of one or more generations, and so the original embryonic 

 condition regained. 



Specific differences do actually exist between the typical embryonic 

 cells of the cambium and of the vegetative apex, and these differences are 

 still more marked in the case of somatic cells which retain the power of 

 producing buds, and in which therefore the germ-plasma is present, but in 

 a dormant condition. 



The fact that an embryonic cell, which normally would develop into 

 a vascular element, may form an epidermal cell when an operative injury 

 causes it to be freely exposed, is proof positive that the special course of 

 development of a particular cell is not the result of any specific peculiarity, 

 but of the interaction of its general properties with determining factors due 

 to its position. The same applies to the rudimentary tissue-differentiation 

 which the cell-segments derived from the apical cell of a moss undergo. 

 Even in the early divisions of the fertilized oosphere which mark off the root 

 from the shoot, the polarity is one of position only and does not involve any 

 inherent specific dissimilarity of the segment-cells. These remain, in fact, 

 of generalized character, so that it has actually been found possible to 

 develop an entire plant of Orobanchc from a portion of the suspensor. 



In the case of the different foliar organs, the courses of development may 

 at first coincide and subsequently diverge, or they may diverge from the 

 first appearance of the leaf-primordium. It is also possible that the 

 determining factors which would lead to the production of a floral leaf 

 might subsequently alter so that a foliage leaf was produced instead, and a 

 zigzag development of this kind probably occurs when a change in the 

 external conditions causes a leafy axis to develop in place of a flower. 



All primordia of generalized character and powers may be termed 

 indifferent, neutral, or indeterminate 1 , whereas primordia or cells which 

 have acquired specific differentiation by the partial suppression or modifica- 



1 Vbchting (Organbildung, I, p. 240; II, p. 36 ; Bot. Ztg., 1895, p. 90) throughout uses the term 

 ' indifferent primordia ' in the above general sense, and it is not easy to see why Goebel (Flora, 

 1895, Erg.-bd., p. 212) should object to it. Goebel (Organography, 1900, I, p. 8) assumes that 

 a primordium is specifically differentiated as soon as it becomes visible, an assumption which has 

 proved to be incorrect. An apparent dissimilarity may be merely the result ol the suppression of 

 certain of the potential powers under the predominating conditions. 



