^Iri'aiST.Tsvo! ] Microscopic Structuve of Plants. 33 



condition be treated with a dilute solution of Iodine and Iodide of 

 Potassium the protoplasm is coloured brown, and the drops of 

 cell-sap can be very distinctly seen. In making observations on 

 the young cells of plants, it is of great importance to select plants 

 which contain httle or no starch. All the observations above men- 

 tioned can be made in the roots of the garden pea, but the quan- 

 tity of starch in the cells greatly obscures the clearness of the 

 demonstration. At the stage now reached, the cell consists of the 

 thin outer cell-wall, the protoplasm with the nucleus, and a few 

 central drops of cell-sap. The nucleus is soon pushed aside, but 

 always remains unbedded in the protoplasm. As the cell grows, 

 the cell-sap occupies more and more space until at length the 

 whole central cavity of the cell becomes filled with cell-sap, the 

 protoplasm forming a layer in close relation to the cell-wall. 

 The nucleus has now been so pushed out of its place that it is 

 apparently adherent to the cell- wall. This can be well seen in the 

 roots and stems of many Monocotyledons. By means of dilute 

 acid you can separate the thin layer of Protoplasm and the nucleus 

 fi.*om the cell-wall. This condition of the protoplasm was described 

 by H. Von Mohl as the Primordial Utricle. The next stage in 

 the life of the cell consists in the almost total disappearance of the 

 protoj^lasm nucleus and cell-sap. The last stage of all is charac- 

 terized by the entire disappearance of cell-sap, only a series of 

 dry cell-walls being left, which can be well seen in the pith of 

 many plants. The Histological Characters of the cell are thus 

 seen to differ at the various stages of its growth. AVe may con- 

 sider the perfect cell as consisting of a cell-wall, protoplasm and 

 the nucleus, and the cell-sap. It is, however, only at those places 

 where growth is actively going on that we find cells presenting 

 these characters ; the great mass of a plant consists of cells, with 

 the primordial utricle and nucleus, or merely of dry cell-walls. 



Having thus considered the cell as being made up of three 

 distinct parts, we must now examine each part a little more in 

 detail. The examination of the cell-wall would lead us to con- 

 sider the various forms which cells assume during the process of 

 growth; these modifications suiting them to their several posi- 

 tions in the structure of the plant, or adapting them to the per- 

 formance of some definite and special function. Besides variations 

 in the form of the cell-wall, the most important change which 

 occurs during the process of growth is thickening. This thickening 

 may be regular or irregular, external or internal. In most cases 

 the thickening is internal to the primary cell-wall, but in a few 

 cases the thickening is external. In the case of pollen grains and 

 spores the inner layer, the Intine or Endosporium, seems to cor- 

 respond to the primary cell-wall, while the Extine or ExosjDorium 

 is an external thickening layer. Almost every variety of external 



VOL. III. D 



