Page:Popular Science Monthly Volume 83.djvu/240

236 are able to hold their leaves and other organs in position. This could not be done without the exertion of considerable pressure, for their delicately constructed leaves and other organs often assume positions requiring a great deal of support. In trees and shrubs there is a large amount of mechanical tissue which supplies the necessary means for supporting the various members.

What is termed the "power of growth" can be determined by learning the amount of weight required to stretch a rapidly growing stem to its original length after the turgidity of the cells has been destroyed by placing them in plasmolyzing solutions, such as a 10 per cent, solution of potassium nitrate. The mean area of a cross section of a stem in millimeters, divided by the amount of weight obtained in grams, gives the number of grams per square millimeter of surface, and, as previously stated, there is usually obtained by this method a pressure of one to three atmospheres or more in the cell for ordinary growth.

While this osmotic pressure is common in ordinary growing organs, it does not necessarily follow that it constitutes the limit, since in the case of the ostrich fern previously referred to it was much higher. When growth is mechanically restricted or the organism has obstacles to overcome, the cell turgescence or osmotic pressure may be greatly increased owing to the resulting stimulus, and this is what occurred in the case of the ferns.

If a cross section of a stem is made and the bark split vertically, a noticeable shrinkage of the bark takes place, demonstrating a difference in tension between the outer and inner tissues. On the other hand, if longitudinal slices are taken from the outside of a common sunflower stem, they will shorten from 1 to 4 per cent, of their length, while the tissues from the center of the stem (pith) will lengthen from 1 to 6 per cent, when removed. It is clear from these observations that the