MOLECULAR STRUCTURE IN PROTOPLASM 



169 



arrangement of molecules is involved in the 

 synthesis of glucose molecules which con- 

 sist of some twenty atoms, while another 

 set ot mechanisms within the same plastid 

 may be building these glucose molecules 

 into visible starch grains consisting of mil- 

 lions of molecules. One is inclined to think 

 of the formation of the starch grain as 

 being on a much higher level of activity, 

 since it is visible, than the formation of a 

 glucose molecule which is far below visibil- 

 ity. This example is one of many which 

 are in progress in definitely localized re- 

 gions, and may serve to accentuate the con- 

 ception of activities within other minute 

 inclusions in the cell. 



In order to give these microscopic struc- 

 tures a place in our comprehension of com- 

 monplace objects, a comparison is made of 

 this cell to a single drop of water such as 

 might hang suspended on the end of a 

 medicine dropper. The drop is about 2 mm 

 in diameter ; its volume then is about 4 cu 

 mm; as contrasted to this the cell may be 

 0.02 mm in diameter and its volume only 

 about .000,004 of one cu mm. In other 

 words, that small drop of water on the end 

 of the pipette is equivalent in volume to 

 about a million cells ; and it may be recalled 

 that each of the million cells is partitioned 

 into functional compartments in which 

 different processes are in progress. "When 

 we try to comprehend experimentally what 

 is going on in these compartments, we take, 

 in our clumsy way, not a drop with its mil- 

 lion cells but a whole beaker full of drops, 

 stir them up, filter, centrifuge and dissolve 

 them in order to get rid of the impurities 

 and then do something with the "pure 

 stuff" which may be precipitated out. Our 

 little compartments are completely wrecked 

 by this time, and whatever is salvaged in 

 its **pure form" is perhaps too often as- 

 sumed to pre-exist as a part of a particular 

 mechanism. 



I have been trying to emphasize the idea 

 of structure within the cell ; but more than 

 that, of structures which involve structural 

 members or units of building materials 

 having different sizes. The discussion pre- 

 sented here is limited to the cytoplasmic 

 region of the cell ; and to the structure, not 



of the cytoplasm as a whole, but instead, of 

 regions and particles within the cyto- 

 plasm. The visible components, when the 

 nucleus and plastids are excluded, are 

 principally small unnamed granules and, 

 in addition, an enormous number of much 

 smaller submicroscopic particles which 

 have been revealed by the ultramicroscope. 

 All of these are included within the fluid 

 of the cytoplasm. 



In this cytoplasmic material in which 

 submicroscopic as well as visible particles 

 occur, activities of one sort and another 

 also take place. The evidence for these is 

 not so clear as for the processes in plastids 

 and nuclei. There is some evidence, how- 

 ever, for allocating a certain amount of 

 hjdrolytic activity to mitochondria and to 

 other very small particles just above this 

 submicroscopic region (Horning 1933). 

 In addition to the activities of the indi- 

 vidual particles they may function also in 

 a collective manner. This apparently col- 

 loidal material may ' ' set " into a particular 

 form when cell division is taking place. 

 There is then, at this time, structure of a 

 sort in which these minute particles func- 

 tion as building blocks. The question 

 arises as to what extent these submicro- 

 scopic particles may function also as struc- 

 tures in which internal localized reactions 

 occur somewhat comparable to the activi- 

 ties in a chloroplast. 



To answer this we must step down to a 

 still lower structural level and examine 

 more carefully these submicroscopic par- 

 ticles. Unfortunately our information be- 

 comes more diffuse and less certain, for 

 methods have not yet been devised for de- 

 tailed studies in this region; however, let 

 us turn to the results of observations which 

 at least show that submicroscopic particles 

 exist in great numbers in the clearest of 

 hyaline protoplasm. I refer to observa- 

 tions made by the ultramicroscope. This 

 instrument shows us a highly magnified 

 section of a Tyndall cone; that is, it mag- 

 nifies the glittering, dancing motes in the 

 sunbeam. Gaidukov (1906; 1910), Zsig- 

 mondy (1914), Bayliss (1920), Price 

 (1914) and Taylor (1925) are names asso- 



