3o8 PATTERNS AND PROBLEMS OF DEVELOPMENT 



factor and influences hormone production; but hormones, once produced, 

 also influence the nervous system. It is perhaps of some interest to note 

 that the hypophysis, apparently a highly important factor in the hormone 

 complex, is, like the chief aggregation of nervous tissue, a development 

 from the higher levels of the chief gradient. 



In some cases transmissive and transportative factors may combine in 

 dominance. A gradient established in the earliest developmental stages 

 may influence direction of transport of chemical substance, as will appear 

 in the following section. A transmitted nervous impulse may set free at 

 the end of its path a particular substance, a "neurohumor," which deter- 

 mines the final effect. Physiological isolation from chemical dominance 

 involves essentially the same factors as isolation from the primitive type 

 of dominance: decrement in concentration or alteration with transport; 

 decrease in production by the dominant region; blocking of transport; 

 alteration of the subordinate part, making it insensitive.' 



DOMINANCE IN PLANTS 



Extended discussion of plant dominance is beyond the present purpose, 

 but attention is briefly called to a few points because of their interest in 

 relation to dominance in animals. Experiments on plant dominance be- 

 gan with the early grower of plants, who learned to prune and trim in 

 such manner that certain results were obtained; and the botanist, by ex- 

 tensive and varied experimentation, has thrown much further light on the 

 problems concerned. The most famihar example of dominance in plants 

 is that of the vegetative stem tip over lateral buds at stem-levels below it. 

 In plants which give rise to lateral bud primordia (potentially new axes) , 

 the vegetative tip of the primary axis of some forms prevents the out- 

 growth and development of these buds, unless inhibited in activity or 

 removed, but its dominance may decrease in the course of the life-cycle; 

 in other forms it may retard their development and determine their 

 growth form as lateral branches. The bean seedling is an example of the 

 first type ; the second type appears in many conifers and numerous other 

 plants. In the bean seedling removal, inhibition of the tip, or blocking its 

 effect results in outgrowth of the previously inhibited buds, those of the 

 uppermost node reacting most rapidly and inhibiting more or less com- 

 pletely those of lower levels. In the conifer removal of the tip is followed 

 by the turning-upward of one or more of the uppermost lateral branches 

 and a change from the bilateral pattern of secondary branching character- 



' See Child, ig2ia; 19246, chaps, x, xi; 19296 for earlier discussions of dominance. 



