A Brief Study of Seedling Growth
By Al Muller
Nature’s miracles are all around us. As plants people we are thankful for them, but more or less take them for granted. Sometimes, our curiosity gets the better of us and we try to look into the heart of a miracle that we become acutely aware of.
While browsing a botany book one evening, I became interested in a particular aspect of plant growth that I considered of special importance. This was because I had recently planted some rhododendron seeds that I purchased from our chapter seed exchange. I would like to share this with others of us who are using our magnifying glasses to check on germination and growth. Since I am not a botanist, what follows is my understanding of this subject, and my observations following germination.
First, however, it should be noted that neither rhododendrons nor azalea (ericaceous) seed require a dormancy period prior to sowing. They may be sown immediately after harvesting.
As we are sowing our precious rhododendron or azalea seeds, some hardly larger than a spec of dust, we take time to try to broadcast the seeds as evenly as possible over the surface of the germination media. But what thought do we give regarding the placement of the seed to provide the proper orientation for normal growth? When we plant bulbs, for example, we take care to orient the bulb so that the point is up making sure that the stem growth will be up and the roots down.
The fact of the matter is, one of nature’s miracles guarantees the proper growth of the seed. There, even though corn seed, as another example, is large enough to orient properly during planting, the farmer would never finish planting a hundred acre cornfield if this were necessary. So, he sows by machine at random, knowing that the plants will grow properly. Also, if you planted bulbs on their side, the stem would still eventually grow up, and the roots down. So it is with our small seeds.
Natures miracle, in this case, is a hormone which is known as auxin (from a Greek work (meaning "to increase’). Auxin’s principal function is to stimulate increases in cell length, especially near stem and root tips. Obviously, this is an extremely complex study, and we could get into great technical detail here. However, for our purpose, we will just take a simple look into the part that auxin plays in orientation of the growth of seedlings.
After germination, the roots and shoots grow in response to the effect of gravity with the aid of auxin. Roots are, by nature, positively geotropic, that is they grow in the direction of gravity. Stems, however, are negatively geotropic, growing opposite to gravity. The growing tip of a root and stem, known as the apical meristem is where the auxin is produced in minute, but sufficient quantities. One of the mysteries of growth is the ability of the auxin to redistribute in its concentration. The auxin flows back from the meristem into the elongating part of the new shoot.
Larger seeds, such as corn, quickly develop a separate root and stem. However, the small rhododendron seed develops a single shoot, which is the beginning of the root and stem. In my observation, the root formation is the first priority. If this is true in a germinating seed which is lying on its side when the shoot starts to develop, gravity causes auxin in the upper part of the shoot to stimulate the cells to grow more rapidly than those across the bottom. This is the physiological means for the shoot to bend and grow down into the growing medium (due to the effect of gravity - hence, positively geotropic.) The opposite end, which will form the stem, then begins a slow vertical growth.
I believe that in this initial phase of development, the stem end straightens any initial bend due to natural auxin migration. However, it is interesting to note, that once the bending occurs in larger plants, and growth continues, the only response to changing orientation of the plant will occur in the area of the newly forming part of the stem. As an example, if a growing plant is laid on its side, only the new growing part of the stem will bend upwards for the reasons described above.
Lastly, purely for reference, a plant (which is placed on a window sill) bends to face the light because of another of nature’s factors known as phototropism. This phenomenon is also controlled by our friend auxin. In this case, auxin is redistributed to the shady part of the stem, causing that side to grow at a faster rate. This is most common in sun-loving species. Most shade-loving species display little or no phototropic responses (important in their selection as house plants.)
My reference books point out that plant physiologists are still at a loss to explain all the complex biochemistry of cell growth, the migration of auxin, etc. This brings to my mind the poem TREES, by Joyce Kilmer, which concludes that "poems are made by fools like me, but only God can make a tree."
In the above text, I made observations and expressed my understanding of this very complex subject, but recognize that there is much more to learn. Therefore, I respectfully invite comments from any authority.
You may email me, Al Muller at: email@example.com
Botany For Gardeners, by Harold William Rickett, NY Botanical Garden. The Macmillan Company
Botany for Gardeners, An Introduction and Guide, by Brian Capon Timber Press