Hi there :) In today's post we'll be having a look at some of the characteristics of grasses and legumes as well as the process of germination, growth and seed production.
Grasses
Grasses have a wider adaptation than any of the other flowering plants and can be classified as cool or warm season species. Cool season species use the C3 photosynthetic pathway while the warm season species use the C4 pathway. Grasses are characterised by there cylindrical jointed stems, their long narrow leaves with parallel veins, their fibrous root system and the fact that they are monocotyledon. A monocotyledon is a plant that has only one embryonic leaf in their seed.
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| Monocotyledon (Left) vs Dicotyledon (Right) source |
Different species of grasses may grow for different lengths of time and they may be known as annuals, biennials, or perennials. Annuals die each year after they have produced seed while in Biennials, this cycle is extended to two years. Perennials produce both vegetative and flowering shoots each year for a few to many years.
Grasses may also grow in different ways. Three growth habits exist:
- Bunch Type (Caespitose) - this produces clumps of grass tillers.
- Stoloniferous - this produces stolons which trail on the soil surface. It has root at the nodes and normal green leaves.
- Rhizomatous - Rhizomes grow beneath the soil surface and the grass has white, small, scale-like leaves.
A rhizome is a stem of a plant that is usually found underground. Most grasses are diploid which means that they have two sets of chromosomes per cell. Some grasses (such as some ryegrasses) have four sets of chromosomes per cell and are known as tetraploid. Tetraploid grasses have a larger cell size, broader leaves and fewer tillers. They also have more soluble carbohydrates and less fibre and experience a greater intake by ruminants.
Important Temperate GrassesSome important species of grasses which we have been asked to become familiar with are listed below:
- Annuals:
- Lolium rigdum (annual ryegrass)
- Lolium multiflorum (Italian ryegrass)
- Hordium lepornium (barley grass)
- Vulpia bromoides (silver grass)
- Bromus millis (soft brome grass)
- Perennials:
- Lolium perenne (Perennial Ryegrass)
- Festuca arundinaciea (Tall fescue)
- Phalaris aquatica (Phalaris)
Basic Structure
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| Basic Structure of a Grass Plant Source |
The basic structure of a grass plant is shown above. These plants are composed of multiple connected growth units called tillers. Each tiller produces roots and leaves.
A grass seed is called a caryopsis and consists of the endosperm, which is a large store of starch, and an embryo. The embryo is composed of the primary shoot (or plumule), the root (or radicle) and a scutellum (the first leaf). Germination involves the uptake of water by the seed and this stimulates respiration, cell division and the secretion of enzymes. These enzymes work to break down the starch in the endosperm into sugars. The sugars pass to the embryo to support the growth of the radicle and plumule.
At the tip of the shoot (the apex), a region called the apical meristem is the source of all the above-ground parts of the plant. The shoot also has nodes, which are the points of attachment of each leaf, that are separated by internodes. Internodes are stem tissue which separate one node from another.
When the apical meristem produces a leaf it also produces an axillary meristem (which are also known as 'buds') which can develop into a new tiller. These buds may also grow into rhizomes or stolons which are important storage organs useful for plant expansion. Each grass leaf develops as a blade connected to a sheath which surrounds the stem above the node. The plant grows from the bottom and pushes 'old' leaves upwards. The number of leaves per tiller remains constant throughout the life of the plant, this means that it will produce a constant turnover.
The reproductive growth of grass plants is stimulated by the length of each day as well as the temperature. During reproductive growth, the internodes of the stem elongate. This causes rapid expansion of the stem which lifts the tip of the shoot above the soil surface. The inflorescence (which is the reproductive structure) develops from the tip of the shoot (the shoot apex).
Legumes
Legumes are dicotyledons and can be annual, biennial or perennial. They are valued for their ability to fix nitrogen. Some important legume species which we need to know about are listed below:
- Annuals:
- Trifolium species (annual clovers)
- Medicago species (annual medics)
- Biserrula pelencinus (Biserrula)
- Ornithopus species (Serradellas)
- Perennials:
- Medicago sativa (lucerne)
- Lotus species (birdfoot trefoil)
After the seeds of the legume are fertilised, it enlarges and the ovary wall develops into a pod. The embryo within the seed contains two cotyledons which enclose the embryo and serve as the energy source during germination. This is because legume seeds contain little or no endosperm. Legumes have hard seeds and hardseededness is a mechanism of seed dormancy that allows the formation of a persistent seed bank. An impermeable layer in the seed coat prevents the uptake of water and germination. The rate of breakdown varies between species and cultivars and is mainly dependent on temperature.
During germination, the seed absorbs water and the root emerges to develop as a simple tap root. This website has a nice diagram that explains some of the features of a young legume plant. The hypocotyl elongates and straightens after penetrating the soil surface. The cotyledons are pulled above the soil surface and open for photosynthesis. The roots continue to grow and start to develop secondary roots. Following this, the first unifoliolate leaf and the first trifoliolate leaf emerge, the main stem then elongates and a leaf is produced at each node. The axillary buds at the cotyledonary nodes form new shoots or branches.
The legume shoots grow the most from the tip of the stems, and the end of branches and stolons, at a place called the terminal bud (or the shoot apex). If the terminal bud is removed, the plant is stimulated to branch out from leaf axils, nodes or the crown.
Leaves develop from primordia (an organ or tissue in its earliest recognisable stage of development) at nodes in the plant. In legumes, cell division and expansion takes place uniformly across the leaflets. Three leaflets are attached to a node.
The flowering of legumes is influenced mainly by temperature and the length of day. The inflorescence arises from a bud either at the tip of the shoot or the tip of the leaf. Most legumes are cross pollinated by insects.
That's it for this post, if you have any questions please feel free to ask in the comments section below :)
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