MOVEMENTS OF IRRITATION. 521 



tissues in the plant organism, it is of importance to make ourselves 

 somewhat accurately acquainted with the rigidity, tenacity, and 

 elasticity of the stereome; this can be done by means of the 

 apparatus depicted in Fig. 179, which I have recently had made. 

 It is throughout very solid in construction. It consists of a 

 board B, on which are fixed the two wooden uprights H and 

 H', about 86 cm. in height, and 14 cm. apart. The structure to 

 be investigated, about 300-400 mm. long, is clamped above be- 

 tween the blocks of wood a, 6, and below between the blocks 

 c, d, by tightening the thumb-screws Sch and Sch'. The scale 

 pan SI, suspended from c and d, takes the weight employed to 

 stretch or rupture the object under examination. To the hook 

 Hk on the block c is attached a silk thread F, which is taken 

 through a perforation running between the blocks a and fc, and 

 bisected by their surface of contact, over the lightly running 

 pulley R, and back through a perforation in a. At its end the 

 thread F carries the weight g to keep it taut. The pulley R 

 carries the pointer Z, whose length is ten times the radius of the 

 pulley. The pointer moves over a scale 6?r, which is divided into 

 millimetres. The extension of the structure under examination 

 is therefore read off on the scale with tenfold magnification. In 

 using the apparatus, blocks of wood must be placed below the 

 scale pan, so that the fall will only be slight if the structure is 

 ruptured. 



We may first suitably employ for examination, as I have done, 

 a strip say 400 mm. in length and 2 mm. in breadth from the 

 middle part of the leaf of Phormium tenax. Also long internodes 

 from rye haulms are suitable. We do not, however, put entire 

 internodes into the apparatus, but strips obtained by splitting 

 them longitudinally into four parts. 



Such a strip having been put into the apparatus, we stretch 

 with a load of 1 kg., determine the elongation produced, and re- 

 move the weight again. If the structure now recovers its original 

 length, its limit of elasticity has not been overstepped. We now 

 repeat the experiment with a load of 2*3-10 kg., till finally, at a 

 certain load, the strip gives way. If we are experimenting with 

 strips of tissue, which, like those from the leaf of Phormium, have 

 bast fibres or sclerenchyma as mechanical tissue, we shall find 

 that, even with a large load, they remain almost completely 

 elastic, while collenchymatous structures, although they may have 

 a considerable amount of rigidity, are only very incompletely 



