ADJUSTMENT TO TEMPERATURE 107 



between surface and volume. The volume grows so much more 

 rapidly that the surface becomes unable to furnish adequate 

 food material, and division becomes necessary as a temporary 

 remedy for this condition. In all cells, division is brought about 

 by a preceding indirect division of the nucleus, i.e., mitosis, though 

 in a number of cases the nucleus divides directly without affecting 

 the cell. All the cells of the embryo, for a time at least, are ca- 

 pable of active division, i.e., they constitute meristem or dividing 

 tissue. Even in the seedling, the greater number of the cells 

 have lost this power, which is confined to a few speciaJ regions 

 during the further development of the plant. Practically all vascu- 

 lar plants maintain meristem at root-tips and stem-tips through- 

 out the life period. With the exception of monocotyledons, 

 woody spermatophytes and many herbaceous ones retain in the 

 stem a special meristematic layer, the cambium. The ordinary 

 shrubs and trees possess meristem in their buds and in the layer 

 which produces the protective cork. In roots the pericycle is 

 persistently meristematic, and the inner layers of the cortical 

 cylinder sometimes possess the same property. The parenchyma 

 of both stem and root for a long time possesses the power to pro- 

 duce meristem, and it regularly serves this function in producing 

 the cambium which connects the bundles of the stem. In flower- 

 ing plants the meristem of all growing tips proceeds from one 

 or more groups of apical cells, while in the mosses, liverworts, 

 and ferns it is derived from the division of a single apical cell. 

 In all plants higher than the simple algse, the stimulus of fer- 

 tilization produces growth. In the carpophytes the action is 

 exerted both upon the egg-cell and one or more of the adjacent 

 cells; in mosses and ferns the resulting growth is usually con- 

 fined to the egg-cell. In the spermatophytes, fertilization initi- 

 ates the development of the endosperm as well as that of the 

 embryo, and likewise often produces a striking growth of calyx, 

 receptacle, or other portion of the fruit axis. Both propagation 

 and reproduction are consequently to be classed as phenomena 

 of growth, and to be considered in connection with it. 



Experiment 35. Regions of growth. Sow fern spores on clean, moist 

 sand, and after they have germinated, observe them from time to time, 

 noting the behavior of the apical cell. Germinate seeds of the horse- 

 bean {Vicia faba), and after the seedlings appear, cut off the root and 

 stem tips, as well as some sections of the upper part of the radicle. 



