268 HISTOLOGY OF STEM OF THE WAX PLANT 



and as the canals grow longer, their ends approach each other, the 

 portions of the cell becoming wedge-shaped, as shown on pi. II, 

 fig. 7, and this would be hard to account for on the theory of interfil- 

 tration, for then as the cell becomes more and more lignified, the inner 

 portion would have to contract or the canals become closed. Now 

 while some of the canals do run together and form one, the majority 

 of them run from the circumference to the center. At last the cell 

 becomes almost entirely closed, with the exception of the canals before 

 mentioned. 



An interesting problem arises in connection with the sclerotic 

 girdle. It is formed in the yoimg stem and incloses a portion of the 

 soft tissues between itself and the wood. Now as the stem becomes 

 thicker, the girdle must increase its diameter. The cells surrounding 

 it keep pace with the enlargement of the stem by the simple act of 

 division. The sclerotic cells do not divide, to my knowledge, for 

 they are lignified almost to the center, and any division could be very 

 easily seen. The problem is, how does it grow ? We know that the 

 tissues of plants are under considerable pressure, as is shown by simply 

 cutting a stem in two, when the fluids contained will immediately 

 exude, and this can be explained only by the supposition that they are 

 squeezed out by the contraction of the tissues surrounding them. 

 This tension will explain the flattened form assumed by the sclerotic 

 girdle in the older parts of the stem, for, as stated above, the compres- 

 sion takes place on the radial line. The difference in shape between 

 the cells composing the girdle and the sclerotic ones near it is very 

 marked, the latter being comparatively roimd. The increase in 

 tangential diameter the cells receive would seem hardly sufficient to ' 

 allow the stem to expand as it does. The following experiment was 

 made. A section was chosen in which the sclerotic girdle had partly 

 formed and in which the cells composing it were almost round. The 

 diameter of the girdle was measured and found to be 2 mm; the 

 circumference was then calculated and found to be 6.2832 mm. The 

 cells composing the girdle were counted and found to be about 209 

 in one case and 226 in another. The former figures give .03 mm., the 

 latter .0288 mm., as the diameter of the cells composing the girdle. An 

 average cell of the girdle was then measured tangentially and found 

 to be .035 mm. in diameter. As the cells are slightly longer in that 

 direction than radially, these figures are quite close, and we may adopt 



