THE CELLULAR CONSTRUCTION OK THE PLANT 27 



threads. These pass for the most part through those thin areas of 

 pit-membrane, where the distance to be traversed is the shortest. 

 Occasionally they may also extend through the thicker regions of the 

 cell-wall. Examples are shown in Fig. 16, A, B. The prevalence of 

 Protoplasmic Continuity, now generally demonstrated for the tissues 

 of Plants, forms a structural foundation for their physiological study. 



B 



Fig. 16. 



Continuity of protoplasm through the walls of plant-cells. A. Cells of the 

 pulvinus of Robinia, after treatment with sulphuric acid to swell the walls, and 

 staining of the protoplasm with methyl violet. ( x 55°-) 



B Cell-wall of a single cell of the endosperm of Lodoicea, showing the pits and 

 the protoplasmic threads, traversing both the thin pit-membranes and the thickened 

 regions of the wall. ( x 400.) (After Gardiner.) 



There is reason to believe that the protoplasm is the seat of physio- 

 logical activity, and since the protoplasm of adjoining cells is con- 

 nected by threads traversing the cell-walls, whole tracts of tissue will 

 be able to share a common life. This leads us to expect that organs 

 will react as a whole under external stimulus, and that though the 

 cell may appear to be an individual structural unit, still each cell 

 takes its place as a constituent of that physiological commonwealth 

 which we call the Plant-Organism. 



While we thus recognise the physiological importance of the con- 

 tinuity of protoplasm through the cell-walls, it should be remembered 

 what circumstance it is that has made it necessary. It is the presence 



