4 STRUCTURE OF THE PLANT-CELL 



Subsequent addition of iodine gives a blue colouration to the 

 dissolving walls, a reaction characteristic of cellulose, of which 

 most thin cell-membranes largely consist. 



It must be realised that the cells just examined, like all 

 plant-cells, are developed in three dimensions, a fact which can 

 be verified by cutting a section transversely across the Onion- 

 scale, when the epidermal cells will appear as flat tabular struc- 

 tures (Fig. i, C). 



As a second example, one of the cells forming the purple 

 hairs (Fig. 2, A) on the stamens of the Common Spiderwort 

 (Tradescantia) may be examined. 1 These show the same struc- 

 ture as those of the Onion, but the cell-sap is here of a purple 

 colour. The nucleus, surrounded by a small mass of cytoplasm, 

 is not uncommonly suspended in the middle of the vacuole, 

 the enveloping cytoplasm being connected with that lining the 

 cell-wall by a number of protoplasmic strands (Fig. 2, B). If the 

 granules in these strands are closely observed, they will be seen 

 to exhibit a continuous streaming movement which can be 

 accelerated by slight warming, but ceases on the addition of a 

 poison (e.g. alcohol), whereby the cell is killed. A temporary 

 cessation of the movement can be brought about by mounting 

 the cells in water to which a trace of some anaesthetic (e.g. chloro- 

 form) has been added. On returning the hairs to ordinary water, 

 however, streaming of the granules is soon resumed. A similar 

 effect is obtained if the cells are mounted in water which has 

 been boiled and subsequently cooled, the result showing the 

 necessity of oxygen for the performance of such movements. 

 The movement is really due to a flowing of the cytoplasm, and 

 this phenomenon betrays the fluid consistency of the latter. 

 No movement can be recognised, however, in the plasmatic 

 membranes, which are therefore probably of a denser character. 

 All living plant-cells display the features above described, 

 but in many cases other structures are present, and of these the 

 commonest are bodies known as plastids. In those cells of the 

 plant which are exposed to the light the plastids become the 

 depositories of the green pigment, or chlorophyll, and are then 



1 If material of the Spiderwort is not available, a good substitute is 

 furnished by the unicellular hairs lining the inner surface of the corolla- 

 tube of the White Dead-nettle, in which,however,the cell-sap is uncoloured. 



