194 The Phenomena of Morphogenesis 



We may distinguish position with reference to the external environ- 

 ment (light, oxygen, chemical stimuli, and many others), with refer- 

 ence to various factors in the internal environment (surfaces, air spaces, 

 conducting strands, and cells previously differentiated), and with refer- 

 ence to the autogenously unfolding and genetically controlled pattern of 

 development. To distinguish these aspects of position is often diffi- 

 cult or impossible. The important fact is that in the organized system 

 specific parts are markedly unlike each other and that these differences, 

 which in the aggregate distinguish the system, may arise from various 

 causes. 



Fig. 8-5. Portion of a regenerating xylem strand in Coleus, showing pattern of wall 

 thickenings laid down along bands of granular cytoplasm. Earlier stage at right. 

 ( From Sinnott and Bloch. ) 



Intracellular Differentiation. The parts of a single cell often show a 

 high degree of diversity. The distinction between nucleus and cytoplasm 

 is present even in embryonic cells, but as the differentiation of the 

 cell takes place, the protoplast may exhibit a wide range of structures. 

 Conspicuous among these are the plastids. In the algae (as Spirogyra), 

 these may be represented by large and often complex chromatophores. 

 Much more minute bodies, the mitochondria, occur and multiply (Soro- 

 kin, 1955; Hackett, 1955). Bodies similar to the Golgi apparatus of 

 animal cells have been reported (Weier, 1932), but their general oc- 

 currence is doubtful. 



