18 INTRODUCTION TO CYTOLOGY 



endosperm, sieve tubes, and thickened parenchyma are actually proto- 

 plasmic in nature. Their resistance to potassium hypochlorite and their 

 arrangement in growing walls suggest to him that they are rather struc- 

 tural constituents of the wall itself. 



Protoplasmic connections have been rather widely described in 

 animals, and are best known in epithelial, muscle, and connective tissues 

 (Fig. 12). In connective tissue they may be broad cytoplasmic exten- 

 sions, giving the tissue the character of a protoplasmic network (Fig. 10), 

 or they may be very fine threads. As in plants, they are the result 

 either of an incomplete division or of a fusion of cell outgrowths. The 

 fact that cells in animals are mostly divided by a process of furrowing 

 indicates that connections, if present, must be for the most part second- 

 arily developed. In his study of living tissues Chambers (1924) finds 

 in the majority of cases no direct evidence of protoplasmic bridges and 

 is inclined to agree with certain other workers in interpreting many 

 reported connections as either fixation artifacts or fibrous differentiations 

 in the intercellular substance, rather than actual protoplasmic strands. 

 Such strands are clearly present, however, in squamous epithelium 

 (Chambers and Renyi, 1925). Some investigators, as will be pointed 

 out in a subsequent chapter, regard the intercellular substance itself 

 as living matter of a special kind. The establishment of protoplasmic 

 connections between blastomeres has been described. ^^ 



The chief significance of protoplasmic connections of all types prob- 

 ably lies in their coordinating function. That plasmodesms serve to 

 transmit stimuli from one cell to another is indicated by their presence 

 in tissues of plant parts known to be particularly responsive to external 

 stimuli. 1^ The effects of mechanical injury appear to be transmitted 

 through epithelial tissue by way of such intercellular bridges according 

 to Chambers and Renyi (1925). Their extensive development in storage 

 tissues, such as the endosperm of seeds (Tangl, 1879; Gardiner, 1897), 

 also suggests that they are in part responsible for the readiness with 

 which nutritive materials are translocated in such specialized tissues. 



The protoplasm of the entire individual is more or less continuous 

 from the beginning of the ontogeny onward. It should not be thought, 

 however, that without such connections there can be no correlation. 

 Mere contact is sufficient for the passage of electrical stimuli, which, 

 as will be indicated below, are recognized as important factors in the 

 development and operation of the body. Cells separated by delicate 

 colloidal membranes with no actual protoplasmic continuity are still 

 able to interact and influence each other's behavior so that the entire 



" G. F. Andrews (1897), E. A. Andrews (1898), Shearer (1906), Whong (1931). 



'* The leaves of Mimosa (Gardiner, 1884) and Dionoea (Gardiner, 1884; Macfarlane, 

 1892); the stamens of Berheris (Gardiner, 1884); the sensitive labellum of the orchid 

 Masdevallia muscosa (Ohver, 1888). 



