148 



A TEXTBOOK OF BOTANY 



[Ch. IV, 6 



Put 



\ 



was thought that the water ascends by capillarity, precisely 

 as oil rises in a wick, or water in a towel ; but experiments 

 have proven that water cannot thus rise in wood more than 

 a few feet. Also it has been held that atmospheric pressure, 

 by which water is raised in a pump to a height of thirty-two 

 feet, would explain it, the greater height reached in trees 



being supposed to 

 result from the fact 

 that intermixed air 

 makes the water in 

 the ducts much 

 lighter than with- 

 out it ; but further 

 study has shown 

 that neither are the 

 conditions in the 

 plant suitable for 

 the operation of 

 atmospheric pres- 

 sure, nor would it 

 suffice in very tall 

 trees. Also it has 

 been argued that 

 the water ascends 

 in the walls of the xylem by a process of imbibition, due to 

 the attraction of wood for water, in precisely the same way 

 that water passes into wood across the grain ; but experi- 

 ments have shown conclusively that the water goes through 

 the cavities, not the walls, of the ducts. Still later it was 

 claimed that hving cells, in wood parenchyma or medullary 

 rays, which accompany' the ducts, act as a means of pro- 

 pulsion of the water upward, each living cell absorbing water 

 from the upper end of one duct and forcing it into the lower 

 end of one higher, somewhat on the analogy of tiny force 

 pumps ; but experiments seem to have jiroven that the water 

 still rises when all living cells are killed by poisons. Finallj-, 



I III I ' I 



Ficr. 103. — Radial section (i.e. parallel TMth 

 a medullary ray) of the Pine of Fig. 102. 



A medullary ray runs across the tracheitis, 

 with the younger and outer end, containing pro- 

 toplasm, on the left. Outermost, on the left, are 

 sieve cells showing sieve plates on the walls ; next 

 is cambium ; then tracheids of the spring wood, 

 showing the characteristic bordered pits ; theu 

 autumn wood containing a resin canal. Com- 

 parison with Fig. 102 will show the construction 

 of a bordered pit, across which extends a thin 

 and flexible plate. (From Cavers.) 



