ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 811 



exceed the former at any moment, the result is the destruction of the 

 molecular reticulum, or, in other words, of the organization. Pro- 

 toplasm differs from other organized bodies in that the grouping of 

 its molecules is undergoing perpetual change, the result of this mole- 

 ciilar activity being the phenomena which we term vital. 



The growth in thickness of cell-walls and of starch-grains takes 

 place, according to Professor Strasburger, by the deposition of succes- 

 sive layers ; in opposition to Nacgeli's view, that the mode of growth 

 was intussusceptive, with subsequent differentiation of layers. Even 

 the surface-growth of cell-walls is not, in his opinion, intussusceptive, 

 but is merely due to stretching. 



With reference to the mode of formation of the cell-wall and of 

 the thickening-layers, Strasburger agrees with the view of Schmitz 

 that the cell-wall is formed by the actual conversion of a layer of the 

 protoplasm, that is, chemically speaking, by the production of a layer 

 of cellulose from a layer of proteid. When a mass of protoplasm is 

 about to clothe itself with a membrane, the peripheral layer becomes 

 densely filled with minute proteid bodies, the microsomata, and this 

 layer then becomes converted into cellulose. The wall of a young 

 wood-cell of Pinus, for instance, is clothed internally with a layer of 

 protoplasm filled with microsomata, which are arranged in spiral 

 rows ; the microsomata then gradually disappear, and the layer of 

 protoplasm is found to be replaced by a layer of cellulose, which 

 presents spiral striation corresponding to the previously existing rows 

 of microsomata, and which constitutes a thickening layer of the cell- 

 wall. In cells the walls of which become much thickened, the whole 

 of the protoplasm may be gradually used up in this way. Again, the 

 wall of pollen-grains and of spores is formed from a peripheral layer 

 of the protoplasm which contains abundant microsomata. Its subse- 

 quent growth, and especially the development of the asperities which 

 it commonly presents, is effected by the surrounding protoplasm 

 which is derived from the disorganized tapetal cells ; this is especially 

 well shown in the development of the epispore of Equisetum and of 

 Marsilia. When an intine or endospore is present, it is produced 

 like the outer coat from a peripheral layer of the protoplasm of the 

 pollen-grain or spore. Further, the septum which is formed in the 

 division of a cell is produced in the same way. The cell-plate, like 

 the peripheral layer of the protoplasm of a young pollen-grain, con- 

 tains microsomata which disappear, and it is then converted into a 

 plate of cellulose. Finally, the successive layers of a starch-grain 

 are produced by the alteration into starch of layers of proteid- 

 substance derived from the starch-forming corpuscle (amyloplast). 



Professor Strasburger next points out that the starch which makes 

 its appearance in the chlorophyll-corpuscles under the influence 

 of light, is derived from the proteid of the corpuscles by dissociation. 

 The formation of this starch is therefore not the immediate product 

 of the synthetic processes going on in the chlorophyll-corpuscles, but 

 only a secondary product. The processes in question produce proteid. 

 Professor Strasburger is inclined to accept Erlenmeyer's hypothesis, 

 that methyl aldehyd is formed in the chlorophyll-corpuscles from 



