342 A TEXTBOOK OF BOTANY [Cn. VI, 12 



our gardens we have double flowers in an extreme abundance, 

 the doubling in some cases being due to the transformation of 

 stamens to petals, and in others to a multiplication of petals. 

 Thus it is plain that no line can be drawn between variations 

 and abnormalities, sports and monstrosities. 



We should now note somewhat more fully the causes of 

 monstrosities, as to which we have little exact knowledge, 

 though some good circumstantial clews. It was once be- 

 lieved that they are mostly reversions to a simpler ancestral 

 condition, but further knowledge has shown that they are 

 usually reversions to a simpler structural condition. They 

 are chiefly due to disturbance in the growth control mecha- 

 nism. The development of any organism and its parts 

 depends upon three sets of factors : First, there is the supply 

 of matter and energy contributed by the metabolism of the 

 plant, and as these are supplied to every living cell, all 

 parts have thus the power and the impulse to grow without 

 dependence upon the others. Second, there is the guidance 

 of the development of the particular parts, exercised in 

 some way by the chromosomes through the cytoplasm, and 

 partly determined by heredity and partly by responses to 

 external stimuli. Third, there is correlation between the 

 different parts of the plant such that the power and impulse 

 of each part to grow far more than it does is kept in restraint 

 and subordinate to the development of the organism as a 

 whole, as witness the case of buds, sometimes forty times 

 more numerous than are permitted normally to develop 

 (page 138). As to the mechanism of this correlation we have 

 as yet no idea, though it is clear that the physical path of its 

 operation lies through the protoplasm which is continuous 

 from cell to cell. Now monstrosities can often be traced to 

 a failure in operation of some one of these sets of factors, 

 but they seem oftenest to result from a failure in the third, 

 caused by mechanical damage to the path of conduction (as 

 in case of burls, page 200) or by chemical paralysis through 

 action of parasites (Witches'-brooms, page 198). When the 



