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 th(.' membrane. The water, however, presses 

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

 increases in size. By this pi'ocess there api)ear continually in the 

 precipitation-membrane extremely fine holes and cracks ; these, 

 however, become closed by new precipitate at the pioment of 

 their ajipearance. Thus, the artificial c(;ll grows continually and 

 uniformly larger until all the gelatine is m combination. The 

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



take i)lace relatively rajiidly, 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- 

 iion, i.e., bythe deposition of new particles 

 between the old ones, or by apposition, 

 i.e., by the deiDosition of particles upon 

 the outside.^ This discussion arose in 

 connection with Nageli's unha])])y com- 

 jmrison of, or rather distinction between, 

 growth in crystals and growth in organ- 

 isms. Latel}' the view has been gradu- 

 ally accepted that both modes lead to 

 the growth of the membrane — the one 

 to 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 eel!, 

 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 ])arallel to the surface, which is 

 visible under high magnifying p(jwers 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 t'xerett' 

 them to the outside continually, exti'nsive solid masses are gradually 

 formed, which in multicellular tissues, where the products cjf 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 stoi'(Ml up as a solid mass in a \'acuole in the 

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

 crystal. Thus, starch-grains in jilant-eeJls, and calcareous needles 



' Gf. ],. 122. 



3. 58. — Cell-wall of a pith-cell of 

 Clematis, showing stratified 

 growth in thickness. (Aftoi- 

 atrasburger.) 



