Exercise I 



THE PROTOPLAST 



I. Introduction. — It is now a well-known fact that protoplasm, 

 which constitutes the physical basis of all life, is organized 

 and "subdivided" in the higher plants into small microscopic 

 units which are termed cells. Usually each living cell, reg-ardless 

 of its position, form or function, consists of a wall which encloses 

 a uninucleate protoplast. Exceptions to this typical condition 

 are furnished by the multinucleate protoplasts of certain fibers, 

 vessels and lactiferous elements. Such coenocytes are of con- 

 siderable theoretical interest with reference to the problem of 

 the origin and significance of the multicellular plant. The so- 

 called "Cell Theory," propounded over a century ago, regards 

 the organism both ontogenetically and phylogenetically as a 

 "cell republic" which has arisen by "the aggregation of a vast 

 number of elementary individuals" or cells (cf. Sharp 1934, pp. 

 20-24). In contrast, the "Organismal Theory" attaches less 

 importance to the septate condition and regards cellular struc- 

 ture as the result of the growth of the organism as a whole. The 

 definitiveness of the cell wall in the tissues of all higher plants 

 doubtless has encouraged the continued wide acceptance of the 

 "Cell Theory" as the more useful concept, at least in any analyti- 

 cal study of plant anatomy. 



The term "cell" was originally applied in 1665 by Robert 

 Hooke to each of the numerous "cavities" observed by him in 

 such material as charcoal and cork. Later, with the discovery 

 of protoplasm, the major emphasis was placed upon the living 

 protoplasmic body and the cell wall was regarded as a "lifeless" 

 secretion of the protoplast. At the present time, however, it 

 seems necessary and justifiable to include hoth the protoplast as 

 well as its wall under the general term of "cell." Evidence in 

 support of this viewpoint is furnished (1) by the apparently 



