CERTAIN GENERAL CHARACTERISTICS 19 



lose membranes are resorbed, and the whole area consists of embryonic 

 growing and dividing cells; but the gradient system persists. Since divi- 

 sion and growth are more rapid in the center of the locus than peripheral- 

 ly, the developing bud grows out from the leaf surface, and the radial 

 gradient system becomes an apicobasal axial gradient system. Figure 3 

 shows a vertical section of a bud after outgrowth has begun. It is evident 

 that the gradient in cell size and cytoplasmic content extends from the 

 free surface proximally as well as from the center peripherally. In the 

 stages shown in Figures 2 and 3 the bud represents the apical region of the 

 new axis. With further elongation the free end becomes the vegetative 

 tip, leaf buds appear in definite relation to it, and differentiation of vascu- 

 lar bundles begins at a certain distance from it. 



In the seedlings of flax {Linum usitatissimum L.) development of ad- 

 ventitious buds can be induced from epidermal cells of the hypocotyl by 

 removal of all buds distal to this region. Five stages in development of 

 these buds are shown in Figure 4. In the earliest stage (upper left) only a 

 single cell has begun to divide; the later stages figured show a general 

 radial and basipetal gradation of increasing cell size, indicating decrease in 

 rate of division from the region of primary activation. In most cases 

 among the seed plants adventitious buds do not arise from epidermal cells 

 but from tissues below the surface, but the visible pattern does not differ 

 fundamentally from that described here. Adventitious buds may arise in 

 callus tissue which develops on wounded surfaces in many woody plants. 

 Here, also, bud pattern is essentially similar to that of the epidermal bud. 

 Roots also develop from buds which originate beneath the surface of the 

 plant body. As in other buds, the apical region develops first, and other 

 parts arise from it as it elongates. 



So far as gradient pattern is concerned, early stages of animal buds are 

 very similar to those of plants. Figure 5, an early stage of amphibian 

 limb buds, is of interest in comparison with the figures of plant buds. The 

 decrease in number of nuclei per unit area from the mesodermal region in 

 contact with the ectoderm suggests an activity gradient of some sort. In 

 the relation between the primary gradient pattern of the bud and morpho- 

 genesis the differences between plant and animal bud development are 

 much like those between embryonic plant and animal development. In 

 the plant bud the apical region usually remains for a time or indefinitely 

 an embryonic growing region, and morphogenesis begins at a certain dis- 

 tance from it. As new material is added to the axis from the tip, each level 

 begins morphogenesis when the critical distance between it and the tip is 



