A blade of grass, if you look closely, has long stripes running from its base to its tip. These are technically the veins containing vascular tissues: xylem and phloem. These veins conduct water and minerals up into the blade and sugar from photosynthesis down into the stem and roots. The arrangement is what textbooks call parallel venation. Each vein enters the broad leaf base independently, and remains parallel and separate from all the others up to the tip of the leaf.
This anatomical feature of grass leaves speaks to the difference between the traditional division of flowering plants into dicots and monocots (of which grasses are perhaps the epitomy). Though dicots are no longer recognized as a single distinct clade, they do exhibit a suite of characters by which they differ sharply from the monocots, and therefore can be referred to informally.
Typical dicot leaves, for example, have a net venation, in which a few veins enter the leafblade via a narrow petiole and then branch repeatedly into a netlike pattern.
These contrasting leaf types are the result of a different pattern of growth. Dicot leaves form as miniatures, with just a few veins, then expand in all directions, adding finer veins between the main veins as they go. Monocot leaves, on the other hand, produce new leaf tissue at the base, which pushes the older part of the leaf upward. The many parallel veins appear when the leaf is young and very short, then new tissues are added to the bottom of each as the leaf gets longer.
The distinctive pattern of growth of the grass leaf is found throughout the monocots, at least in their seedlings, and it appers to have begun in the ancestral monocot as an adaptation to growth from an underground stem. The tips of the monocot leaves become mature and stiff when they are very short, so as to be able to push through the soil, while the softer, growing bases are nestled below ground among the bases of older leaves.
Many monocots retain this subterranean existence with their primary stems taking the form of rhizomes, bulbs, and corms. Those that don't, such as single-stemmed palm trees, still have seedling leaves that push up from below the ground.
The leaves of most dicots, on the other hand, are adapted to form at the tips of exposed twigs, with the miniature versions forming within buds, then expanding rapidly to full size.
A number of dicots, such as this aquatic pennywort, have petioles that can extend through basal growth to lift their blades above water. Something like this may have preceded the first monocots. |
It is therefore not certain whether monocots were initially adapting to aquatic conditions or to harsh seasonal conditions. They may have been preceded by aquatic dicots that also have the ability to extend their leaves upwards through basal growth of their petioles (leaf stalks). One theory is that in the ancestral monocots, basally growing petioles eventually lost their conventional leaf blades, and the petiole became wider to become the new kind of blade.
From basic monocots with underground rhizomes, bulbs, and corms, many specialized forms have evolved, including tree-like members of the palm, pandan, and Dracena families. Epiphytic orchids, aroids and bromeliads have rhizomes that creep along tree limbs. Banana plants use basal growth to lengthen their cylindrical leaf sheaths into a false stem, while Egyptian papyrus lifts a globular crown above water through basal growth of its flowerstalk. And finally, the giant grasses known as bamboos rapidly reach tree height by regions of basal growth in each of their internodes.
Thanks for the refresher... got here through a link from an article shared by NCSE
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ReplyDeleteThank you so much for writing this article. :) I was wondering why monocots have parallel veins. I figured that it was an apomorphy. Branched veins are typical for the dicots, which is a paraphyletic group among the flowering plants. That is interesting how monocots use parallel veins to better push out of the ground, from a protected stem. Grass uses that, and it can take a lot of abuse: grazing, trampling, blowing, burning and freezing. I think an ancestor of the monocots was probably aquatic. The most basal monocots form a paraphyletic group of aquatic plants. My favorite is the arum. I like it for its beautiful and exotic flowers. A more subtle characteristic is the leaves. The veins are parallel, but their is still a lot of branching from the basal vein. It seems to be a transition from dicot to monocot. Maybe plants like the arum have partially buried stems. The aquatic origin does explain the growth. Interestingly there is convergence with another plant group, the water lilies. These plants also have an underground stem, which the leaves and flowers come out of. As the name suggests, water lilies are adapted to an aquatic environment. However they are not to be confused with true lilies, which are monocots.
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