Exercise I 



LIVING CELLS (1) 3 



Stain by adding a drop of iodine-potassium 

 iodide solution (I2 + KI, 0.01 A/ each). Cover 

 with a cover slip and look for the deep purple 

 color that indicates the presence of starch. 



COLONIAL ORGANISMS 



Spirogyra, a green alga 



Spirogyra is commonly referred to as pond 

 scum. It tends to float on the surface in many 

 fresh-water streams and ponds and is recognized 

 by its bright green color and slippery feeling. 

 Place a few filaments of Spirogyra on a slide in 

 a drop of water. Cover with a cover slip. Under 

 low power select a group of cells with regular, 

 spiral, green chloroplasts. Examine under high 

 power. Note the pyrenoids in the chloroplast. 

 These are associated with starch formation. 

 Where is the nucleus and how is it held in place? 

 Sketch a cell showing the various structures 

 you see. 



Test for starch by adding to a strand of 

 Spirogyra on a slide a drop of iodine-potassium 

 iodide solution as above. Note on your sketch 

 the structures that stain most deeply with 

 iodine. 



Volvox, a "colonial" green flagellate 



A Volvox colony may contain several thou- 

 sand cells, embedded at the surface of a gela- 

 tinous sphere, with the flagella — two per cell — 

 directed outward. The cells are interconnected 

 by delicate strands of protoplasm. (You will see 

 the arrangements better if you stain by drawing 

 a drop of methylene blue under the cover slip.) 

 Moreover the cells vary in size, shape, and func- 

 tion. For both reasons this is more than a 

 simple collection of cells; it represents a genuine 

 approach to a differentiated, multicellular or- 

 ganism. Sketch a colony. Do not try at this 

 time, however, to see very much of the structure 

 of individual cells. You will be able to do that 

 better with closely related unicellular green 

 flagellates. 



OTHER ALGAE 



Your instructor may also have other algae in 

 the laboratory for optional study, perhaps an 

 example of such a stonewort as Nitella; perhaps 

 a diatom, one of the golden algae; or a desmid; 

 or such a unicellular green flagellate as Chlamy- 

 domonas. 



What is the simplest cell you have seen today? 

 What would you say of the simplicity of the 

 organism of which it is a part? The complexity 

 of a multitissued organism is achieved through 

 the specialization, and concomitant simplifica- 

 tion, of its individual cells. Does specialization 

 always imply simplification? 



A NOTE ON THE 

 COMPOUND MICROSCOPE 



Realms of dimension 



In a development that stretched over nearly 

 three centuries, the compound microscope 

 brought biologists into a world of new dimen- 

 sions. Their dissections had previously been 

 concerned with the gross anatomy of tissues and 

 organs. Now they could penetrate to cellular 

 anatomy. This involved a leap in dimensions 

 of about 1,000 times, roughly from the level of 

 millimeters to that of microns (10~^ cm). A 

 cell in a multicellular plant or animal is usually 

 one to several microns in diameter, though some 

 algae are enormously larger, and bacteria in 

 general very much smaller. The limit of resolu- 

 tion in visible light, that is, the separation at 

 which two points in the object are seen as two 

 rather than as a single blob, is about 0.2 micron. 

 No details finer than this can ordinarily be dis- 

 tinguished, no matter how fine the instrument. 



Recently the electron microscope has per- 

 mitted a further leap in dimensions of approxi- 

 mately another 1000 times, from microns to 

 millimicrons [1 mju = 10~' cm = 10 angstrom 

 units (A)]. This has brought us from micro- 

 scopic to ultramicroscopic anatomy, from the 



