Ginkgoaceae

 

Ginkgo

 

This monotypic genus is the only survivor of a once widespread and dominant Family of ancient plants – they derive from the early evolution of the Gymnospermae. It represents an early phase in the evolutionary development of woody flowering plants. It is an evolutionary anachronism to have survived to the present time.

 

The Maidenhair Tree (Ginkgo biloba) from China, although rare in its native habitat (indeed if it is recognisable and still exists), is nevertheless a very ornamental tree and a great survivor of ecological changes and man-made disaster. It develops into a tall, long-lived, deciduous tree with a characteristically upright central leader and readily recognisable fish-tail shaped leaves. The leaves turn to brilliant shades of yellow in the autumn.

 

The species is dioecious and the flowers are produced in early spring just before bud burst and pollination is by wind. The female trees develop the autumn maturing fruits in the one season. The fruits consist of a single-seeded drupe-like structure - about 30mm long and 25mm broad. When ripe the flesh is particularly evil-smelling with an acrid clinging odour which owes much to the presence of compounds of butyric acid – for this reason it is prudent to plant only male flowering trees in amenity situations. Such selections are inevitably propagated vegetatively and although cuttings can be rooted successfully, they grow slowly. Propagation is therefore usually achieved by grafting and for this purpose seedlings will be needed as understocks.

 

The seeds are extracted from the fruits by maceration, fermentation in warm water and then rubbing (wear rubber gloves to avoid contamination with the smell) – the dross can be decanted off leaving the seeds to be washed in detergent, then rinsed and surface dried.

 

Storage of the seed can be achieved under water conserving conditions, at low temperature - when acceptable levels of viability can be retained for several months. Longevity is, in general, therefore only short term – a few months at best. Storage at warm temperature causes a rapid deterioration of viability as the oily food reserves quickly become rancid.

 

The seeds are individually about 25mm long, ovoid and somewhat flattened in one plane with a lengthwise suture around the broadest axis; the ends are slightly pointed. The seed coat is made of a thin, hard, horny material which although not impermeable to water, does protect the seed from physical damage especially in the gut of any animal which may have the ability to eat the fruit. The seeds are a creamy-beige colour when fresh. The seeds are variable in size from lot to lot which, in general reflects the warmth and suitability of the particular growing season which the parent tree experiences. The soundness of the seed is also a reflection of the season – the suitability for pollination in the spring and the nature of the growing season for food reserve production. In warm locations seed production is normally prolific and of high quality.

 

Germination is initially delayed by the presence of a partially differentiated, immature embryo and subsequently by a low level chilling requirement. The degree of embryo maturity at the time of seed dispersal is a function of the temperatures experienced during the late summer - consequently the level of embryo development at dispersal determines the length of time needed to mature the embryo. This is therefore not a constant feature. To achieve maturation of any lot, whatever its status, warm storage of the imbibed seed is therefore set at 56 days in excess of 15ºC - regardless of the initial level of immaturity.

 

Although a few seeds will germinate if sown and maintained at a warm temperature over the long term, a successful and reasonably synchronised emergence, in an acceptable time scale, can only be achieved if the seed is subjected to an adequate chilling regime: the most effective treatment is achieved with a period of storage of the imbibed seed at 5ºC or less for 42 days – but even so the response may still not be completely uniform.

 

The most effective method for dealing with the cleaned seed sample is to:-

a) soak the seed for 18 to 24 hours in warm water to ensure full imbibition;

b) extend the sample in four times its volume of moist vermiculite in a suitable bag or container;

c) store at <15ºC for 56 days;

d) transfer to refrigeration, for chilling, at >5ºC for 42 days and then

e) transfer to incubation at 20ºC.

 

The sample should then be regularly monitored and when it can be seen that the seed coat is splitting and the radicle is beginning to emerge, the seeds can be sown individually one to a liner container - as each seed is ready. This treatment provides the most reliable system for achieving as uniform emergence as possible. The seed should be sown on its ‘keeled’ side to give the best orientation for efficient and swift emergence.

 

The seedlings are not frost tolerant.