Propagation by Seed
Propagation by Seed
The word propagation literally means to increase by natural means, and this opens the flood gates for a discussion on what is ‘natural’ and what is ‘artificial’. Setting aside this provocative argument there is, in the whole sphere of greenhouse culture, great scope for examining every facet of propagation under conditions where success is more likely than in the widely varying out-of-door conditions which prevail in most countries.
The living plant
Before propagation can be fully understood it is necessary to consider the basic botanical make-up of the living plant. Flowering plants are divided broadly into two main groups: dicotyledons to which most broad-leaved plants belong, and monocotyledons which consist of narrow-leaved plants includingand most . The basic difference as far as propagation is concerned lies in the existence of a cambium layer in dicotyledons which is lacking in monocotyledons. The cambium cell layer exists between the vital xylem and phloem necessary for conducting water and nutrients in the plant, and its task is to replace worn out cells and act as a source for regenerating the whole plant. Cambium cells are in fact the vital key to some aspects of successful propagation from miscellaneous sections of plant; it is mainly only the cambium cells which are able to bring about physical union between plants in grafting and give rise to the formation of new roots when rooting cutting. It is fairly obvious therefore that one cannot propagate monocotyledons in the same way as they do not possess a cambium layer, which imposes definite limitations in propagating methods (with the exception of Dracaena).
In addition to monocotyledons and dicotyledons, a further grouping of plants relates to their longevity, there being three well-known groups:
- Annuals – These arise only from seed and die after forming seed, though not necessarily in the one calendar year, eg calendula, annual asters, clarkia, and many more (many annuals are of course half-hardy, living only a part of their lives out of doors).
- Biennials – These live for two seasons or, to be more specific, part of two seasons; in the first they prepare for flowering and in the second they flower, when thereafter they produce seed and die, eg foxgloves.
- Perennials – Plants in this group carry on flowering and producing seed for a great number of years, storing up their food in roots or stems, which enables them to survive the dormant period, although they may die down out of doors. Woody perennials have a permanent shoot system which usually does not die back to the ground (eg shrubs).
Annuals and biennials are rarely propagated by methods other than from seed, whereas perennials can be propagated either from seed, cuttings, or other parts of them such as roots, stems leaves, and cells.
It must not be imagined that the gardener can choose the method of propagation he prefers, as plants exhibit considerable temperament in respect of how they increase themselves. A cutting from a large soft bush such as an elder may be extremely difficult to propagate, and likewisewhich has a very hard stem. Snips of coleus or tradescantia (wandering Jew or Sailor) root in a few days, whereas many tender ericas (eg Erica hymealis) can be very tricky to root. The leaves of many plants can seldom be induced to root, whereas others such as those of Begonia rex produce roots with ease.
The eccentricities of seed are well known, where due to cross pollination and other causes the germinated offspring is different in colour or form from the parent plant. But though it may be assumed in the majority of cases that any portion of a plant’s anatomy which can be successfully rooted will produce a plant of identical character and pattern of growth to the plant from which it was taken (setting aside ‘sporting’), this is not always the case. Cuttings taken from mature plants generally produce plants which flower earlier than those produced from young plant cuttings. Where cuttings are taken from juvenile types of seed-raised conifers, the juvenile form persists. Straggly cuttings taken from older conifers will produce straggly plants, unlike those produced from young short-jointed cuttings from the terminal shoots. There would also appear to be a need to select particular types of chrysanthemum, carnations or other cuttings for stem propagation.