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If we take two pieces of bast (which is nearly pure cellulose), 

 one twice as thick as the other, suspend them, and hang weights 

 at their free ends, we find that these pieces resist a very consid- 

 erable stretching force. If we continue to add weights, a point 

 IS reached where even the thicker piece will become appreciably 

 ger than it was before. If the weight on both pieces is 

 the same, the thinner piece elongates twice as much as the thick 

 piece, showing that this substance resists such a force in direct 

 proportion to the thickness of the pieces. When each of the 

 two pieces has attained its new length it is at rest, and the thin 

 piece stretched to its new length equilibrates the thick piece in 

 Its less elongated condition. If now we take such a piece of bast, 

 say only ten times as long as thick, we find that a very small 

 lorce suffices to bend it to a slight extent, such as is the case in 

 the guard-cells when the stoma is opened. Not that bending 

 and stretching are different in their nature, but it is because a 

 very slight stretching on one side, and a corresponding slight 

 compression on the other, differing, of course, with the relative 

 thickness of the piece, is sufficient to permit such sUght bending, 

 see, therefore, that a small part of the force necessary to 

 stretch the bast through a considerable distance is enough to bend 

 It through a very considerable arc. To illustrate : A strand of 

 Drac(2na bast, one millimeter in cross-section, is not stretched 

 beyond the limit of perfect recovery, or elasticity, by anything 

 less than a weight of seventeen kilogrammes, while to bend such 

 a thread of about ten millimeters length a very small part of one 

 l^'ilogramme would be sufficient. 



Suppose now that we have a tube made up of such bast-like, 

 or cellulose, material, and let its wall be of equal thickness 

 throughout. If this tube is filled with liquid under a high pres- 

 sure it will become distended, quite firm and rigid, but it will 

 have no tendency to bend in any direction, since its wall will be 

 stretched equally throughout If, on the other hand, the wall is 

 "ot of equal thickness, but is thickest on one side and thinnest 

 on the side opposite, the thickness varying gradually between 

 these extremes, this wall can not be stretched equally by a hy- 

 Prostatic pressure from within, since this power is the same 

 on all sides, and the wall, like the pieces of bast, will be stretched 



We 



