THE CELLULAR CONSTRUCTION OF THE PLANT 31 



cell-wall. Examples are shown in Fig. 19, A, B. The prevalence of 

 Protoplasmic Continuity, now generally demonstrated for the tissues 

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

 There is reason to beUeve that the protoplasm is the scat of physio- 

 logical activity, since the protoplasm of the several cells is connected 

 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 



Fig. 19. 



Continuity ot 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 550.) 



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.) 



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 

 of the cell-wall itself. The encysted state of the cell is a feature of all 

 advanced types of Plant-Organisation. It was probably secondary 

 in origin, and it is amply justified by the strength and protection 



