i 7 4 AQUATIC STEMS [CH. 



stained sections, and partly as a survival from the old days of 

 the 'imbibition theory,' when the ascent of water was sup- 

 posed to be due to some mysterious property peculiar to the 

 lignified membrane. But it is now universally recognised that 

 water travels in the cavities of the vessels and tracheids rather 

 than in the walls. What part then does lignification play in the 

 ascent of water? It must be remembered that the water-con- 

 ducting elements are dead and empty, and that in terrestrial 

 plants they often contain air, which is more or less rarefied, 

 and is thus at low pressure. These dead elements are generally 

 in contact with turgid living cells, which exert a strong pressure 

 against their walls. From the point of view of the ascent of 

 water, the only function of the lignified walls of vessels and 

 tracheids appears to be to prevent their being crushed by the 

 neighbouring living elements. The way in which tyloses force 

 themselves into vessels through the defenceless, thin places in 

 their walls, gives some idea of the pressure which living cells 

 are prepared to exert. In hydrophytes, however, the circum- 

 stances are very different. The vessels, instead of frequently 

 containing rarefied air, as in the case of land plants, are pre- 

 sumably more continuously full of liquid, and are therefore 

 less liable to be crushed and obliterated by the surrounding 

 living elements. The conduction of water is not, in their case, 

 conditioned by the possession of armoured walls. There is every 

 reason to suppose that the non-lignified conducting elements 

 of a submerged plant may be as effective in raising water as the 

 woody vessels of a terrestrial tree ; that water does, as a matter 

 of fact, travel freely in the non-lignified xylem spaces of the 

 submerged Potamogetons has been shown by experiment 1 . 



Elongated, submerged stems, unless they grow in perfectly 

 still water, must be subjected to some amount of tension from 

 currents. It is probably more than a mere coincidence that the 

 vascular system of aquatics is so often condensed into a central 

 strand, recalling the central cylinder of roots and of climbing 

 stems, both of which are organs subjected to pulling forces. 

 1 Hochreutiner, G. (1896); see pp. 261-263, Chapter xxi. 



