G'day Sean,

In article ,
ose (Sean Houtman) wrote:
From: (Phred)

I can verify that in the long awned species of Stipa, it is definitely used
in
drilling the seed into the soil. Since the awn is genuflected, it holds the
caryopsis at an angle, and vibration from the wind pushes the seed into the
soil, aided by the hairs on the callus. When the seed has been pushed in far
enough, the awn breaks off and blows away. It is perfectly adapted for this
function. Other grasses certainly use the awns in other ways, not all of
them
apparent.

Yeah, lots of tropical grasses with awns use them to "plant
themselves" too -- but it can also be a response to wetness (or just
humidity?). When wetted, they twirl around until stopped by
vegetation or debris or whatever, then the forces involved act to push
the seed itself into the soil.


I wouldn't be at all surprised if this is the case in wetter climes. Here in
New Mexico the Stipas do have a number of turns on the awn, but the seeds are
dispersed during the dryest time of year and a rotational drilling caused by
moisture isn't going to happen very often.

Ah, different perspectives! Now *I* almost mentioned that I thought
this "self planting" mechanism was developed here in grasses
especially adapted to our seasonally dry tropics with a view to
burying the seed as soon as the first storm of the new season arrived.

Of course, it could happen earlier with out of season rains (or even,
in much of the dry tropics, with heavy dews) and the seed would be
prevented from premature germination in such cases through one
or another of the usual dormancy mechanisms.

But if we're talking *dispersal* then that's another matter, and I'm
not sure what help the awns would be in that sense -- except perhaps
mechanical entangling in hair, fur, or feathers. (But I can say that
the seeds of black spear grass [_Heteropogon contortus_] don't need
the *awns* to become painfully embedded in your flesh! The hairs on
the tip of the "seed" seem to do the job quite well enough if they've
got a bit of purchase on something like your socks. And some of the
three-awn spear grasses [_Aristida_ spp.] are worse! )


Cheers, Phred.

--
LID