32 PROTOPLASM 



nerve fibers serve as lines of communication between tissues. 

 More significant is the fact that animal cells are in direct contact 

 with one another; protoplasm touches protoplasm (the inter- 

 vening cell membranes are of protoplasm and therefore do not 

 nullify our statement). But plant cells are separated by heavy 

 cellulose walls. If a tip of the leaf of the sensitive plant Mimosa 

 pudica is touched, within a few seconds all the leaflets are closed, 

 and the petiole drops. Communication has taken place from 

 cell to cell. One can imagine several ways in which this might 

 happen; for example, an electrical stimulus may have traversed 

 the electrolytic (salt) solution which bathes the cells and per- 



a b 



Fig. 21. — Protoplasmic connections (plasmodesmata) between the cells of, 



a, the persimmon, Diospyros (from C. J. Chamberlain) ; b, the moss Madotheca 

 {from B. N^mec). 



meates the cellulose walls. But such a nonliving method of 

 communication would not seem to be very satisfactory. One 

 would suspect the presence of a vital, a protoplasmic connection 

 of some sort. A very careful examination shows that such a 

 connection does exist between cells. Vital contact between 

 plant tissues may be maintained by delicate intercellular proto- 

 plasmic strands known as plasmodesmata (Fig. 21) which pene- 

 trate the heavy cellulose walls. How general these are in plants is 

 unknown ; indeed, their very existence is doubted. The presumed 

 protoplasmic strands are said by some to be but pits or canals 

 in the cellulose wall, but L. G. Livingston supports the earlier 

 work of Nemec and makes it quite clear that plasmodesmata do 

 exist. Comparable intercellular connections of large size indis- 

 putably exist through the sieve plate of sieve tubes (food-con- 

 ducting vessels). Similar protoplasmic bridges occur in the red 

 algae. These give vital continuity between cells for the trans- 

 mission of stimuli. To know that protoplasmic connections 

 between cells exist in certain cases is of little help when we learn 

 that apparently it is not so necessary as it would seem. Boysen- 



