STEMS FLEXIBLE AND RIGID III 
siderations go; such shafts—as we may observe by 
watching the Oar-weed (Laminaria) on an exposed 
coast—are effective under very arduous conditions. 
Those Seed Plants which, evolved on land, have later 
returned to the water, such as the Pondweeds (Pota- 
mo geton), have often redeveloped a stem of a similar 
kind—a flexible shaft possessing a sufficient tensile 
strength. The specific gravity of such plants does not 
exceed that of the medium in which they are immersed, 
and the stem has not to support the weight of leaves 
and branches. It is, therefore, not surprising to find 
that the longest, though by no means the bulkiest, of 
all plants, are found in the sea. Some of the Oar- 
weeds (Macrocystis) of the southern and western 
Oceans attain lengths which have been estimated at 
500 to 1,000 feet; but these gigantic Seaweeds are 
nevertheless slender plants, suspended lightly in the 
water. But after the colonization of the land by the 
aquatic flora numerous serious problems had to be 
encountered and solved before plants in an aerial 
environment could rise boldly into the air. Extremes 
of temperature unknown in the water had to be faced. 
Along with a greatly increased loss of water owing to 
the presence of air and direct sunlight, the area over 
which water might be absorbed became largely 
reduced, the roots alone being now available. The 
whole weight of branches and leaves and fruit had to 
be borne by the stem, not only in calm but in storm. 
No wonder that to meet these conditions, or to avoid 
such extremes as were avoidable, aerial stems often 
display great complexity and diversity of structure 
and form. From the mechanical standpoint the tall 
stem is especially interesting on account of the beau- 
