86 PROTOPLASM 



isolated as to leave it attached to the body by a strand through 

 which blood flows. With the tissue in Ringer's solution, Hirsch 

 studied the cellular activity for a period of thirty hours, particu- 

 larly in reference to secretion and cytological changes as the result 

 of treatment (with pilocarpin, etc.). 



Still another method for studying tissue is that developed by 

 the anatomist E. R. Clark. The lobe of a rabbit's ear is per- 

 forated, making a hole about an inch in diameter which is covered 

 with two thin plates, one of glass and one of mica, held very close 

 together. During the process of healing, the new tissue grows 

 between the plates, forming a very thin layer so that the growing 

 cells and blood vessels can be seen with great clarity as through 

 a window. 



Plants. — Haberlandt was mentioned in the introduction of this 

 chapter as the originator of the thought that isolated cells would 

 grow in culture. He was a botanist and naturally tried to prove 

 his idea by growing plant cells in a nutrient solution, but he failed. 

 The culturing of plant cells should be, and every one at first 

 expected that it would be, much easier than the growing of animal 

 cells. Plants, in nature, show a very ready tendency to save 

 themselves from destruction by developing new shoots and form- 

 ing new individuals under most unfavorable conditions. A tree 

 trimmed until only the trunk remains may burst forth with new 

 shoots at the next spring. A fence post set when green may 

 sprout. Cuttings of many plants grow very readily. The life 

 plant Bryophyllum has the extraordinary capacity to develop 

 a new plant from each notch in a leaf when the leaf falls to the 

 ground. Many tropical ferns proliferate in this manner. Mere 

 cutting of the veins of a Begonia leaf will cause a new plant to 

 arise at each cut. Why do such cells, which exhibit rejuvenating 

 powers far exceeding those in animals, not grow in tissue culture? 

 In a sense they do, but not as undifferentiated tissue. They 

 always form fully developed organs. It is as if the plant cell were 

 determined to fulfill its purpose in life and form an entire plant 

 or not grow at all! 



A near approach to plant tissue culture is the growth of extir- 

 pated root tips. Such work has been done by Robbins, Kotte, 

 White, and Gautheret. If the tip (1 cm.) of a young root from 

 a sprouting seed is cut off and put in a Pfeffer salt solution with 

 organic material, it will grow for weeks or months under favorable 



