108 ' GENERAL PHYSIOLOGY 



drop. At the surface of the latter the tannin is united with the 

 gelatine, and thus the continual apposition of new layers leads to 

 the thickening of the membrane. The water, however, presses 

 into the interior of the drop, so that this constantly swells and 

 increases in size. By this process there appear continually in the 

 precipitation-membrane extremely fine holes and cracks ; these, 

 however, become closed by new precipitate at the moment of 

 their appearance. Thus, the artificial cell grows continually and 

 uniformly larger until all the gelatine is in combination. The 

 formation and growth of the membrane, which in the large drop 



take place relatively rapidly, proceed 

 very gradually in the small living cell. 



In botany a fruitless discussion has 

 been going on for a long time over the 

 question whether the cellulose-membrane 

 of the plant-cell is formed by intussuscep- 

 tion, i.e., by the deposition of new particles 

 between the old ones, or by apposition, 

 i.e., by the deposition of particles upon 

 the outside. 1 This discussion arose in 

 connection with Nageli's unhappy com- 

 parison of, or rather distinction between, 

 growth in crystals and growth in organ- 

 isms. Lately the view has been gradu- 

 FIG. 58. ceii-waii of a pith-ceil of ally accepted that both modes lead to 

 EV < Sg >l "$ the growth of the membrane-the one 

 strasburger.) o growth in surface, the other to growth 



in thickness. If the protoplasmic body 



of the cell itself is enlarged, the membrane is extended. In the 

 process, as a rule, no actual cracks appear, as in the artificial cell, 

 but as a result of the extension the spaces between the single 

 particles of the membrane become wider and larger, so that new 

 particles of protoplasm can enter in. But, on the other hand, the 

 stratification of the membrane parallel to the surface, which is 

 visible under high magnifying powers and with increasing thickness 

 becomes constantly more distinct, shows that increase in thickness 

 by apposition is also present (Fig. 58). 



If the cells in their metabolism produce substances and excrete 

 them to the outside continually, extensive solid masses are gradually 

 formed, which in multicellular tissues, where the products of the 

 individual cells blend together, form the so-called intercellular 

 substances, such as in cartilage and bone (Figs. 59 and 60). But 

 the substances are not always excreted at once to the outside ; in 

 many cases they are stored up as a solid mass in a vacuole in the 

 cell itself, particle after particle being added to them as in a 

 crystal. Thus, starch-grains in plant-cells, and calcareous needles 



1 Of. p. 122. 



