280 GROWTH 



amount of substances having a powerful attraction for water, and 

 this brings solutions into the cell containing nutritive compounds. 

 The pressure of the turgidity set up enlarges the cell by stretch- 

 ing its membranes, and the increase in volume is followed by a 

 corresponding increase in the cytoplasm as a consequence of the 

 rapid assimilation that ensues at this period. The nucleus may 

 follow this increase slightly in certain specialized instances in 

 which the final fate of the cell is that of a glandular secreting 

 element, but in general constructive tissues the nucleus does not 

 increase, or even maintain the size shown immediately after di- 

 vision. The increase of the cytoplasm continues until the cell 

 has reached its full differentiation, or adult form. An accom- 

 panying enlargement of the vacuoles has ensued. 1 The length of 

 life of the cell after maturity shows the greatest variation. Pa- 

 renchymatous cells of the pith or cortex may remain alive and 

 active for a long term of years and the medullary rays of the beech 

 are known to live for more than a centuiy in some instances. 

 The tracheids may live two or three years, while the vessels are 

 perhaps shorter lived. In general it may be said that the length 

 of life of tissues, the major functions of which are mechanical, is 

 comparatively brief. 



357. Size of Cells. Fit a compound microscope with an eye- 

 piece micrometer, that has been calibrated for the combination of 

 lenses with which the instrument is fitted, and ascertain the exact 

 dimensions of a number of cells in various tissues. Compare the 

 parenchymatous cells of different organs of the same plant. Es- 

 timate the actual size of the guard cells of the stomata, and the 

 size of the opening which they regulate. Compare root-hairs 

 from different species. 



358. Average Size and Bate of Growth of Some Unicellular Or- 

 ganisms. Cultivate any convenient species of Spirogyra in a 

 small glass aquarium at temperatures between 8° and 22° C. 

 Mount a few filaments on a glass slip and plaee on the stage of 



1 Minot, C S. On certain phenomena of growing old. Proc. A. A. A. S. 1890. 

 Minot, C. S. On heredity and rejuvenation. Amer. Naturalist. 30 : 1, 89. 1896. 



