Gneater (neater) than all get out!

Visits to our teaching greenhouse still manage to deliver surprises even thought I have been doing it regularly for years.

This is something the Phactor had never seen before: the ovulate strobilus (cone) of Gnetum (knee-tum). Hmm, that's pretty funny, telling you how to pronouce a silent G with a silent K.

Gnetum is a tropical liana, and that's one of the reasons I've never seen strobili before. They're usually way, up, there. The closest I've come before is the seeds, mature ovules, which are about 4 cm long. They have a fleshy, reddish outer seed coat that attract and reward an animal disperser. Finding these seeds on the rain forest floor means that somewhere far above where the vine clamors along in the canopy, a strobilus just like the one above got pollinated.


The strobilus consists of several whorls of small bracts (modified leaves) above which is a whorl of ovules. Ovules are not eggs so their name is a misnomer. Ovules are jacketed, indehiscent megasporangia. The single megaspore produced develops into a haploid female (a gametophyte) who then produces an egg. The jacket prevents the pollen access except through a small hole. The ovules shown here are ready to receive pollen. The ovule has exuded a a drop of sticky liquid out through this small opening and any pollen grains (endosporic males) that adher to the surface of this pollen drop get pulled inside the ovule when the ovule reabsorbs the liquid.

This is not the work of a passive female. She reaches out, grabs passing males, and pulls them into her lair.

Now if the foregoing description wasn't a tipoff, what makes Gnetum so Gneat is that this is a gymnosperm, a relative of conifers, albeit a fairly distant one. Yet as you can see, this vine has broad leaves, and it has vessel elements in its wood, both features usually associated with flowering plants. And of course there is a certain flower like quality to the strobilus.

Do flowering plants share a common ancestry with Gnetum? The jury is still out. For many years data suggested that the answer might be yes, but most recent studies suggest this strange plant is more closely related to conifers, and not in a gymnosperm lineage having a common ancestry with flowering plants.

Plant of the week (eon?) - Cooksonia

This picture of Cooksonia is pretty remarkable. Not so much for displaying this plant's simple beauty, not even because the picture is actually in focus, but because this plant has been extinct since the Devonian some 380 million years ago.

This fossil is looking particularly good because it is a model on display in the Darwin House at the Royal Botanical Garden at Kew.

Cooksonia is significant because this is the first land plant sporophyte to show apical branching. As the axes grew, the apex divides into two equal axes, a type of branching called dichotomous. Fossils show axes of Cooksonia branching thusly 1 to 4 times. Each axis then terminating with a sporangium. This appears to be a way of getting the maximum number of dispersable offspring from a single fertilization event.

Cooksonia is also significant because at the time this was the tallest plant of its day. These axes towered some 5 centimeters (nearly 2 inches) above the substrate. It is also the earliest appearing plant to have vascular tissue, xylem, although the oldest specimens found in Silurian sediments may not have possessed true xylem.

This plant is the first step on the way to trees, and it did not really take too long for the first trees and forests to appear (near the end of the Devonian).