CHAPTER IV 



PHYSIOLOGICAL CHARACTERISTICS OF 

 AXIATE PATTERNS 



BEFORE turning to the experimental analysis of morphogenesis, 

 , "determination," and differentiation, it seems desirable to devote 

 some attention to certain physiological characteristics of axiate 

 pattern. Since agamic development and reconstitution of isolated cells or 

 pieces occur in many organisms in a definite relation to the pattern of the 

 parent, some of the physiological features of adult pattern, as well as 

 those of early developmental stages, are of interest in this connection. 



PLANTS 



A few points concerning plants are noted because they suggest certain 

 similarities of plants and animals as regards general pattern. In some 

 fifty species of axiate algae examined with various agents, differential sus- 

 ceptibility decreases basipetally in axes with apical growing tips, at least 

 in the younger parts of the axis (Child, 1916c, e; 1917a, h; 1919/). In the 

 older parts of the thallus irregularities often appear. Multiaxiate forms 

 with regularly arranged bipinnate or radial branches show the basipetal 

 gradient in each branch and in the system as a whole; that is, the suscepti- 

 bility of the growing tips of branches decreases from branch to branch 

 basipetally. In axes consisting of a single series of elongated cells the 

 progress of cytolysis can be observed, not only from cell to cell, but along 

 each cell from the distal to the proximal end. The whole bipinnate thallus 

 of the alga, Bryopsis, is a single cell; but the gradient is the same as in sim- 

 ilar multicellular axes (Child, 1916c). In axes with a basal growing region 

 susceptibility decreases from the base acropetally; the frond of the kelp 

 Nereocystis (Child, 1919/) and hairs of the Fiicus thallus (Child, 1917a) are 

 examples. Unpubhshed data on the differential susceptibility of Vohox to 

 a large number of agents (see Appendix III, p. 735) show decrease from the 

 pole of the growing region, the posterior pole in locomotion. 



Polarity of the egg of the alga Fucus can be determined by various ex- 

 ternal differentials (see pp. 423-25). Light is probably the usual deter- 

 mining factor under natural conditions, but there is evidence that the egg 



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