268 Postelstia 
strongest in the immediate sublitoral zone the 
tips are kept worn very short so that the plant 
presents very little surface to the waves and 
consequently the strain on the holdfast is 
sight and it is not torn loose. But where the 
wave action is less violent the tip is not nearly 
so rapidly frayed and consequently attains a 
greater length, while the strain on the hold- 
fast remains nearly constant. The anatomical 
means by which this equilibrium is maintained 
is extremely simple. As im: all the kelpseime 
growing point is situated at the junction of 
the stipe and lamina. From this region new 
tissue is progressively pushed farther and far- 
ther out on the lamina by the formation of 
still newer tissue behind. When first formed 
it is tough and tenacious but with age its strength 
gradually weakens so that the tip of the lamina 
is able to withstand but a small strain, while 
the basal portion is quite strong. Thus the 
length of the plant is kept balanced with the 
force of the waves so as to prevent its being 
torn out by the roots as is the fate “of moa 
other kelps. 
