522 PHYSIOLOGY OF GROWTH. 



elastic, and therefore remain permanently elongated after being 

 stretched. To define accurately the rigidity of a strip of tissue, 

 we must express it in terms of 1 sq. mm. of stereome area. If, 

 e.g., the mechanical tissue in a transverse section of a leaf of 

 Phormium, when magnified thirty times,* occupies a surface of 

 900 sq. mm., the actual area of the stereome in the transverse 

 section is 1 sq. mm. If the strip of Phormium ruptures at a load 

 of 15 kg., then the rigidity of 1 sq. mm. of Phormium stereome 

 would be expressed by 15. Frequently, however, there are very 

 considerable difficulties in determining the real area of the 

 stereome in transverse section, even approximately. In many 

 cases we have to be content with rough estimates. 2 



Our object is best attained by sketching thin sections from the 

 weakest part of the object, e.g. from the place where the rupture 

 took place on loading, under slight magnification (say thirty- fold), 

 and then measuring the area of the mechanical elements on the 

 sketch by means of the plani meter (to be obtained, together with 

 directions for use, from J Kern, Aarau, Switzerland, for 45 mk.), 

 or by means of millimetre paper. If necessary, the relation of 

 lumen to wall- thickening may be taken into consideration. In 

 making the measurements solid xylem strands are to be included 

 in the result. 



In many cases, particularly in testing the rigidity of collen- 

 chyma, it is advisable to observe the following method of pro- 

 cedure for determining area in transverse section of the mechanical 

 tissue. We draw exactly, under high magnification, a large 

 number of collenchyma cells, using for the purpose good writing 

 paper, which we may assume to be approximately of the same 

 thickness at all points. We now determine the weight of the 

 paper covered by the drawing, cut out the cell lumina by means 

 of a knife, and weigh the network of paper left. Comparison of 

 the two weights gives the ratio of the total area to that of the cell- 

 walls. If we now sketch under slight magnification the outlines 

 of the collenchyma strands of a transverse section of the object, 

 and determine the corresponding area, we can at once estimate the 

 total area of the wall substance of the mechanical tissue. Good 

 material for investigation of the rigidity of collenchyma is 



* To determine exactly the magnification employed, we sketch on the paper 

 by means of the camera lucida the lines of an objective micrometer divided 

 into 0-Oi mm. We now measure the distance between the lines by means of 

 a millimetre scale, and can then at once deduce the magnification. 



