TRANSMISSION OF STIMULI. 47 



function thenceforth it is to serve as passages for air and water; thus the con- 

 tinued presence of the protoplasts is no longer advantageous. On the other hand, 

 in the second class of vessels the lateral walls of the cells, which have coalesced 

 to form them, exhibit no thickening, but are soft and delicate, and resemble 

 flexible tubing. These tubes, moreover, are not deserted by their protoplasts; but, 

 after the coalescence of a number of cells into a single duct has taken place, the 

 protoplasts in the cells are themselves merged together, and the entire tube is 

 then occupied by an uninterrupted mass of protoplasm, which generally persists 

 as a lining to the wall. 



As the initiation and construction of cell-walls are the work of the living proto- 

 plast, so also is their removal. The home it has made for itself the protoplast can 

 also demolish — either partially or completely. But this demolition is preluded by 

 the importation of particles of water into the portions of the wall which are to be 

 destroyed. The introduction of water brings the wall into a gelatinous condition; 

 the cohesion of its constituent particles is loosened, little by little, and at length 

 completely abolished. 



4. COMMUNICATION OF PROTOPLASTS WITH ONE ANOTHER 

 AND WITH THE OUTER WORLD. 



The transmission of stimuli and the specific constitution of protoplasm. — 

 Vital Force, Instinct, and Sensation. 



THE TRANSMISSION OF STIMULI AND THE SPECIFIC CONSTITUTION 



OP PROTOPLASM. 



As has been already intimated, the breaking down of individual cell- walls and 

 the formation of the various pits, sieve-pores and fine canals in thickened mem- 

 branes, m the manner described in preceding pages, are processes of great import- 

 ance to the life of protoplasts. In the first pla,ce, many of the resulting structures 

 are the means of preserving the -possibility of intercourse with the outside world. 

 In a space inclosed by evenly thickened walls, the absorption of air, water, and 

 other raw materials from the environment would be very difiicult if not impossible; 

 the protoplast inside would soon lack the provisions needful for further development, 

 and would at last die of starvation, drought, and sufibcation. But the little win- 

 dows, whether open or closed by thin permeable membranes, enable it to supply 

 itself with all necessaries of life. Another advantage is derived, in the case of many 

 of these structures, inasmuch as the protoplasts on occasion escape through the open 

 doors and settle down in some other part of the cell-colony, where they are able 

 again to make themselves useful. Lastly, one of the most important benefits of all 

 is due to the fact that mutual intercourse between protoplasts, living together as a 

 commonwealth, is rendered possible by the canals which join them together. And 



