Hi, welcome to Taiga Bonzai, we receive many questions on ‘Plant husbandry’ for example, ” If we plant a seed it will probably germinate, but it would be good to know a little more science on the subject for better understanding.” Okay, we will discuss these issues in the next 4 articles, but as extensive research has already been written (and we will point out the appropriate articles) the discussions will be in brief.
Introduction – germination is defined into two categories Epigeal (above ground) and Hypogeal (below ground) as a seed germinates the first structure to emerge from most seeds is a root from the embryonic called a radicle, this primary root is referred to as a taproot. Smaller lateral roots (secondary roots) arise from the taproot which in turn produce even smaller lateral roots (tertiary roots) these serve to increase the surface area for water and mineral absorption.
The above images show the stages of germination from the radicle to the first true set of leaves and needles respectfully. Cotyledons are the first leaves produced by plants, but they are not considered true leaves and are sometimes referred to as ‘seed leaves’, because they are actually part of the seed or embryo of the plant. These seed leaves serve to access the stored nutrients in the seed feeding it until the true leaves develop and begin photosynthesizing.
Root growth – roots grow in length from their ends only, the very tip of the root is covered by a thimble-shaped root cap called the calyptra, which protects the growing tip as it makes its way through the soil. Behind the root cap lies the apical meristem here cells are produced, some are added to the root cap, but the majority are added to the region of elongation, which lies just above the meristematic region. Above this lies the region of maturation where the primary tissues of the root mature, completing the process of cell differentiation that actually begins in the upper portion of the meristematic region. (shown below)
Aerial roots – some roots called adventitious roots arise from an organ other than the root, for example from a stem or leaf. These adventitious roots often referred to as aerial roots can hang long distances before coming into contact with the soil or remain dangling in the air. Some of these including the Screw pine and banyan do assist in supporting the plant in the soil, aerial roots are the primary means of attachment to non-soil surfaces such as buildings, rocks and other plants for example. The Ficus watkinsiana family Moraceae (strangler fig) named for their pattern of growth upon host trees, which often results in the host’s death.
A number of other specialized roots exist among vascular plants for example. Pneumatophores an aerial root specialising in gaseous exchange are commonly found in mangrove species that grow in saline mud flats. These are lateral roots that grow upward out of the mud and water to function as the site of oxygen intake for the submerged primary root system.
Other root systems – the roots of certain parasitic plants are highly modified into haustoria, a rootlike structure that grows into or around another structure to absorb water or nutrients, mistletoe and members of the broomrape family are good examples of this. Many plant roots also form intricate associations with mycorrhizal soil fungi, a number of non-photosynthetic mycoheterotrophic plants including the Indian pipe rely exclusively on these fungi for nutrition.
Root functionality – the primary tissues of the root are from outermost to innermost, the epidermis, cortex and vascular cylinder, the epidermis is composed of thin-walled cells and is normally only one cell layer in thickness. The absorption of water and dissolved minerals occurs through the epidermis, a process enhanced in most land plants via the presence of root hairs – slender tubular extensions of the epidermal cell wall that are found only in the region of maturation.
The absorption of water is achieved via osmosis, which occurs because (a) water is present in higher concentrations in the soil than within the epidermal cells, where salts, sugars and other dissolved organic products are contained. (b) The membrane of the epidermal cells is permeable to water but not to many of the substances dissolved in the internal fluid. These conditions create an osmotic gradient, whereby water flows into the epidermal cells, this flow exerts a force called root pressure, that helps drive the water through the roots.
The cortex conducts water and dissolved minerals across the root from the epidermis to the vascular cylinder, then transported to the rest of the plant. The cortex also stores food transported downward from the leaves through the vascular tissues, the innermost layer of the cortex consists of a tightly packed layer of cells called the endodermis, which regulates the flow of materials between the cortex and the vascular tissues.
Why no tap root? – In bonsai many practitioners remove the ‘tap root’, but the ‘tap root’ enables stability and water absorption so why remove it? The following deciduous species have rather large tap roots Oak Quercus, Black Walnut Juglans nigra, Silver Fir Abies alba and White Mulberry Morus alba. Coniferous species contrary to popular belief do not have long tap roots, their lateral roots and tertiary roots spread outward and grow downward which gives stability however, there are some exceptions including the Long Leaf pine Pinus palustris that have large tap roots.
In order for these and many other species of tree to become bonsai the roots have to be pruned and the more vigorous the root growth the more pruning is required. In Japan and China young trees are planted in deep pots to encourage root growth and after a few seasons they have their tap roots removed to allow the lateral and tertiary roots to develop and thicken; these roots if near the base of the trunk are the potential nebari.
Root damage – many plants will survive and recover from root damage providing the damage does not exceed 1/4 of the total root zone. Most of the important feeder roots of trees or shrubs are within the upper six inches of the soil and if damaged, uptake of water and nutrients is restricted reducing growth. In addition, root damage may take months or even years to progress, and it is during this period where problems begin which can cause symptoms of decline or death depending on the situation and how much damage occurred.
One of the biggest problems when root pruning bonsai is the lack of care taken, we have witnessed countless instances where the root ball is attacked with 2, 3, and 4 pronged instruments. The roots are basically ripped apart causing irreparable damage and as stated if more than 1/4 of the total root ball is damaged chances are that the tree’s health will diminish for some considerable time and this is where it is susceptible to attack from pests and disease.
Root pruning is an important factor in bonsai horticulture and should not be attempted half heartedly. At T.B. we have some large bonsai (Omani dai class) and these do take considerable time to re-pot. When teasing out the root ball a blunt single root hook is used that does not cause any damage. After which the roots hanging down and separated can be pruned accordingly with sharp shears nonetheless, others will use instruments that they prefer.
This brief discussion on germination and in particular the functionality of a plant’s root system may lead to a better understanding of its importance, more on root pruning can be found in articles 08 ‘Styling, wiring and pruning’ and 54 ‘Summer pruning’. Until next time, BW, Nik.