STEALTH SCIENCE
Plant Morphology and Anatomy 101 Courtesy: By Jon Houseman - Jon Houseman and Matthew Ford, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=35811226
So, now that we kind of understand what plants are made of, how do they grow? The growth simply involves the division, expansion, and differentiation of the cells we just discussed. The cells stretch, expand, and change form into the required type of cell (to make varying organs), depending on the plants’ current physiological needs or environmental influences. This cell division in the stems and roots occurs in the tips of apical meristem, while behind this zone of division, the newly formed cells grow larger (expansion and elongation) as they differentiate into various tissue types (dermal, ground and vascular). These become the different organs such as leaves or flowers. Primary growth describes this development of roots and shoots, but what about the secondary progress as seedlings develop into thick, woody trees?
As the cells increase in number and quantity, the leaf expands in size. Marginal meristem growth occurs on the side of the leaf (but from the base of the leaf in monocots), and progression is suppressed in the region that becomes the petiole. There are different types Photomicrograph of a dicot leaf. of leaf forms (dorsiventral, isobilateral or A-Lower epidermis, B-Lower palisade mesophyll, centric), but most common are bifacial leaves C-Upper epidermis, (dorsiventral). These have a structural difference D-Upper palisade mesophyll, in the upper and lower epidermis relative to the E- Spongy mesophyll, F-Leaf vein. number of stomata and trichomes present. This Scale=0.2mm. accounts for the top receiving the most direct sunlight and exposure to the elements, while the underside of the leaf can provide more structure and exchange gases efficiently into the air. The mesophyll (the layer beneath the epidermis) within a leaf is generally differentiated into palisade or spongy. Interestingly, isobilateral leaves (such as eucalyptus) have symmetrical epidermis and mesophyll tissue, but also have oil glands housed within the centre mesophyll layer, which is what gives us the delicious scents from eucalyptus oil!
Woody Dicot Stem: One Year Sambucus cross-section, 40x magnification During the first year of growth, the epidermis is stretched laterally by the expansion of secondary xylem, phloem and cambium.
Courtesy: Berkshire Community College Bioscience Image Library
This lateral development occurs from activity in two secondary meristems. The Vascular Cambium adds layers of vascular tissue, and the Cock Cambium replaces the epidermis with secondary dermal tissue (bark, cork etc). By repeated divisions of cambial initials, more xylem and phloem are added as the vascular cambium moves further from the centre of the stem. This primary and secondary growth is always occurring simultaneously, but at different locations within the plant. However, the primary growth is restricted to the youngest areas of the plant (tips of the roots and shoots), whereas the secondary growth becomes layered, producing wood. As the stem develops and provides a structurally supportive highway of food and water for the plant, more leaf tissue is required to create additional energy. When a leaf develops, a primordium is first noticeable at the surface of the apical meristem. A primordium is defined as an organ or tissue in its earliest recognisable stage of development.
The internal anatomy of a leaf is usually related to the environmental habitat of the plant and the requirements for endurance. Land plants can be categorised into four morphological groups based on the water availability and the dryness of the air in their surroundings. Hydromorphic plants live in a watery environment and usually have stomata on the upper surface of the leaf (such as water lilies). Conversely, xeromorphic plants live in dry conditions and have evolved thick cuticles with sunken stomata to survive. Epiphytic plants are commonly known as ‘air-plants’ and survive in tropical climates with high humidity and rainfall, living attached to other plants or objects and absorbing nutriment from the atmosphere. The most common plants are mesomorphic, which covers a fairly broad range of different environmental conditions. Their leaf veins (vascular bundles) permeate the ground tissue of the dermal system, giving us a beautiful range of leaf patterns. The anatomy of mesomorphic leaves is designed to function optimally for water uptake and gas exchange in photosynthesis, but we will talk more about that in Topic 4: Photosynthesis and Phloem. Hopefully, this quick article has given you an insight into the structure of plants and their different organ features. The most important feature to understand for all of our flowering or fruiting plants is their accessibility and usage of water. In the next issue, we will look further into this crucial relationship: the transpiration stream and how plants can perform miracles with H2O! 3
BIO Founder of Indicated Technology Pty Ltd, Tom is a certified horticulturalist and paid consultant working in the Australian medical cannabis industry. After finishing studies in production horticulture (hydroponics) and plant biology; Tom has spent the past 6 years working in the protected cropping space. Tom is passionate about sustainable yet economic cultivation methodologies and also teaches cannabis cultivation as part of university and private education programs. Tom is also the Communications Manager for Stealth Garden wholesale supplies. 69
