5th May 2014 is the start of Term 2 and we have been learning about Plant Science, in this case Botany:

We start off with plant anatomy, including cell anatomy and physiology. I find it fascinating and interesting to see that plants and animals are so similar on this scale. The main difference between plant and animal cells are the cell walls, vacuoles and chloroplasts in plant cells. Cell walls and vacuoles give plants their structure, as they do not have shells or skeletons as animals do. Chloroplasts is the specialised cells where photosynthesis takes place. This is the most vital metabolic process that plants do, as it is what gives them their energy. As horticulturalists we are always wanting to find ways to control plant processes, we either want to maximise or minimise photosynthesis, depending on whether we are wanting to encourage growth (during spring and summer) or limit growth and metabolic processes (during harvest). It is vital to have a good understanding of what is going on in the plant in order to better understand how it will react to certain environmental conditions.

This is simply an overview of what we have learned.

We are also concerned with cell division in plants, which occurs through the process of mitosis, which occurs in the meristems of the plant. Meristems are different from other plant cells; they have thin walls, remain young and have large nuclei and lots of small vacuoles. They come in three main types: the apical meristems, which are in the shoots and roots and allow growth upwards and downwards. Lateral meristems, found in the stems/trunks of plants; these allow growth outwards and are the vascular cambium and the cork cambium. There are also intercalary meristems, which are common in grasses, these are unusual in that they appear in mature cells, are usually found in monocots and allow growth in response to damage such as that from grazing animals or lawnmowers.

The vascular cambium is particularly important when it comes to grafting and budding, as the cambium layers must meet up if strike is to occur. The vascular cambium is only present in woody dicot plants. Thus it is not possible to graft herbaceous monocot plants as they do not have a cambium layer, their vascular bundles are scattered throughout the stem, not arranged in a nice ring as with woody plants. Even large tree-like monocots such as palms and our own Cordyline australis do not have a cambium layer.

Primary growth is growth to the length and height of a plant. It occurs at the tips of stems and roots; also leaf axils.

Secondary growth is to plant thickness all over, stems and roots. It takes place in the vascular and cork cambiums.

Buds are very important for trees and plants growing in climates where there are freezing temperatures or periods of drought. Winter buds keep water in and cold out of the new shoot, which is inside. They are covered with protective leaf scales. They are formed in response to shortening day length. You can get leaf or vegetative buds, flower buds or mixed buds which contain both flowers and leaves.

Cherries have simple buds, separate flower and leaf buds. Flower buds tend to be rounded, leaf buds tend to be pointed:

Apples have mixed buds. The two types look very different:

Leaves are the main source of photosynthesis for the plant, they also transpire and respire. Sunlight can penetrate up to 1mm into the leaf. Leaves are covered by a cuticle, made of cutin, a waxy substance that protects the epidermis. They have veins which transport minerals and water. They are made up of a stem or petiole, which turns into the mid-rib of the leaf blade. The margin is the leaf edge and is important for ID of the plant. Some simple leaves also have lobes and sinuses, where the leaf bulges in and out, as in a maple leaf. Most leaves look very different on the top than from the underside and the leaf itself is different at the cellular level top to bottom as can be seen in the diagram below:

Photosynthesis occurs in the pallisade mesophyll cells mostly, in a special organelle called the chloroplast. They have 1000s of them. Light that penetrates deeper than this is used by the spongy mesophyll cells. They also contain chloroplasts but not as many. They also have large intercellular spaces to allow the rapid diffusion of gases such as carbon dioxide.

Leaf veins contain xylem, on the inside of the leaf which transport water and minerals and phloem, which take the sugars made in photosynthesis down to the roots. All plant parts contain xylem and phloem. In the lower epidermis one tends to find the stoma which are pores for the exchange of gases from the environment to the plant and vice versa. They open in daylight and close up at night to preserve water loss. Thus plants only photosynthesise during the day (when the sun is up) but will respire 24 hours a day and transpire too if conditions are right, hence why the stomata close up at night.

Photosynthesis is:

sunlight + carbon dioxide + water + chlorophyll produces: oxygen + glucose + water + chlorophyll is a catalyst which remains unchanged by the process.

Respiration: Plants get energy from glucose, if they do not use it at the time, it is stored as starch in the roots. Glucose can move around the plant going to where it is needed, starch is immobile. When the plants need energy again, they convert the starch back into glucose and transport it to where it is required. All organisms respire 24 hours a day, this takes place in the mitochondria, it is also temperature dependant, so that’s why we cool fruits and flowers quickly after they are harvested. Respiration also requires oxygen, so many places have a controlled atmosphere storage room, where the oxygen levels are taken down from 21% to 2%

glucose + oxygen produces carbon dioxide + water + energy

Transpiration: Plants lose water, usually through the stomata in the leaves, 90% of water loss in plants occurs through the leaves. The xylem are continuous pipes which extend throughout the whole plant. There are 4 forces which act on plants to cause transpiration. Water is uptaken through the roots by the process of osmosis. Water  molecules have a tendency to stick together thanks to hydrogen-bonding, this enables water to act as one mass. The narrow capillary-like tubes of the xylem create a meniscus, with water adhering to the sides of the tubes. At the top of the plant, in the leaves, as transpiration occurs out of the stoma, the pull of the water molecule leaving the stoma, creates a suction, which pulls the next molecule to the stoma.

Transpiration depends on wind, humidity, temperature and light. Transpiration is at its highest rate on hot, dry, windy weather. The process also cools the plant down, which is why sitting under a tree is much cooler than sitting under a shade sail. The process also moves sugars, hormones, minerals around the plant and it helps to maintain the cell water content.

Roots are vital to the plant: They absorb water and nutrients, they conduct water and nutrients to the stem, they provide anchorage to the plant and food storage. There are two types of root systems: tap roots, favoured by dicots and fibrous roots, favoured by monocots. There are also prop roots which are adventitious roots (similar to what are produced when we propagate from cuttings) which sit above the ground and provide extra anchorage.

Roots also have apical and lateral meristems and have a root cambium. A root cap protects the young root tips as they push their way through the ground. Many roots also have root hairs, elongated epidermal cells; they are very delicate and cannot handle heat and light, so when we transplant a plant it needs to be done quickly to protect the root hairs, if too many are damaged, the plant will not survive the transplant. Root hairs only live 1 or 2 days then new ones are formed. Their function is to increase the root surface area and absorption. It is possible to transplant even quite large trees.

Roots grow a long way out from the plant, some tap roots can go very deep but root flare is most important for anchorage. As roots extend so far out from the plant, you need to be aware that inappropriate herbicide use can cause die back, killing all or part of the plant. Root depth is determined by genetics, soil oxygen, soil compaction, the age of the plant and the style of propagation used.

Water and nutrient uptake: Plants need water to dissolve minerals and chemicals, for chemical reactions (such as photosynthesis), for support (in the vacuoles), to increase their size, to replace the water lost in transpiration and to cool their leaves. Water moves into the plant in the roots by a passive process known as osmosis, however nutrients are much bigger than water and cannot pass across the semi-permeable membrane by osmosis, they need to be actively transported across, which requires energy. All molecules tend to spread out as evenly as possible and water is no different. It will move from areas of higher water content to lower water content in order to spread the molecules out evenly. This process is called osmosis. Water content outside the roots tends to be higher than the water in the roots, as the root water is a solution of sugars and nutrients. However this is not always the case. If someone puts too much fertiliser around roots, it can cause reverse osmosis, whereby water leaves the roots as the water content outside the root is now lower than inside the root. This can cause the plant to die if it is not corrected quickly through the addition of more water to the roots.

Plants need 13 essential nutrients to grow and when we do soil tests we are looking to see which of these is missing (if any) and replace them via fertilisation. These nutrients are: nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, boron, iron, manganese, molybdenum, copper, selenium, cobalt and zinc.

Mycorrhizae are soil fungus that have a symbiotic relationship with the plant. The fungi attaches directly to the roots, they help absorb water and nutrients. Each plant species have a different species of fungi related to it. The fungal hyphae can either cover the root or even grow into the root. Some of these mycorrhizae fungi are edible and highly sought after by humans. These include Perigold black truffle, bianchetto, shoro, porcini, saffron milk cap, matsutake and burgundy. It is possible to buy trees infected with these hyphae, so you can grow your own fungi 🙂

Some roots have nitrogen fixing bacteria associated with them. These plants are known as legumes and they all have pods. Although the atmosphere is 78% nitrogen, no one can use the gas in this form even though plants and human need nitrogen. Root nodules are infected with Rhizobium bacteria, which can fix up to 300kg per hectare per annum! See Veggie Garden for a picture of root nodules. Inside the nodule it is quite pink, as they contain a form of haemoglobin.