138 The Phenomena of Morphogenesis 



spore but such a prothallium will develop only where there is an en- 

 vironmental gradient by which its polar axis is established. Such results 

 emphasize the fact that neither genetic constitution nor environment 

 alone controls the development of organic form, but an interaction be- 

 tween them. 



Polarity is also to be found in the microspores and pollen grains of 

 higher plants, though here it is not easily open to experimental analysis. 

 The spore axis, as indicated by the orientation of the division of the 

 spore nucleus, has a constant relation to the planes of division of the 

 pollen mother cell. 



Fig. 6-14. Origin of polarity in a single cell. I, unpolarized spore of Equisetum. II, 

 beginning of polarization as shown by changed positions of plastids (chl) and nucleus 

 (k). Ill, first nuclear division. (After Nienburg.) 



POLARITY IN PLASMODIA AND COENOCYTES 



In larger and multinucleate protoplasmic units, notably coenocytic 

 forms, polarity finds a somewhat different expression than in uninucleate 

 protoplasts. Thus in the plasmodium of Plasmodiophora brassicae, as re- 

 ported by Terby (1933), the axes of the many nuclear division figures lie 

 parallel to one another, indicating that the whole mass of protoplasm has 

 a uniform anisotropic orientation, though here without a polar axis. This 

 parallelity is also found in the first sporogenic division but disappears at 

 the second one. In most plasmodia, however, this simple sort of polar 

 behavior seems not to be present. 



True coenocytes show some remarkable examples of organized systems 

 where there is pronounced differentiation of parts but no cellular parti- 

 tions in the cytoplasmic body. Conspicuous among these are the algae 

 Bryopsis and Caulerpo. 



Bryopsis has an axis from which "leaves" come off in a pinnate arrange- 

 ment above and rhizoids below. This polar organization can be com- 

 pletely reversed if the plant is held in an inverted position. The leaves 

 then produce rhizoids and the rhizoids, leaves ( Noll, 1888; Winkler, 1900; 

 and others ) . It now seems probable that a different relation to light rather 



