Coniferales

 

Taxaceae

 

This family, which until recently had been regarded as a separate Order (Taxales) within the Gymnospermae, has, as a result of the assessment of the evidence provided by modern DNA analysis, been reassigned into the Coniferales as there are not sufficient differences to justify the separation - despite the significant morphological distinctions.

 

The Family is typified by being dioecious and by the development of the fruit as a single seeded, fleshy aril with the seed containing an immature embryo.

 

The Taxaceae contains five genera – Taxus (an important and widespread genus of cultivated trees and shrubs); Torreya (a small genus of similar looking trees and shrubs – not widely cultivated in the UK climate) and the little known Amentotaxus (four species in China – one of them hardy), Pseudotaxus (one hardy species in China but not normally found in cultivation) and Austrotaxus (one species found in New Caledonia).

 

Taxus

 

The Yews (Taxus) constitute a geographically widespread group of plants with a circumpolar distribution in the Northern Hemisphere. The taxonomists have treated them variously as eight species – each with a fairly distinct geographical niche – to one all encompassing species – Taxus baccata. They are evergreen bushes to small, dense, spreading trees with non-resinous wood.

 

The flowers are produced in the early spring and the fruits mature in the late summer to autumn of the same year. The fruit is a scarlet, fleshy, upright, cup-like aril containing a single seed. These should be picked by hand before they are taken by birds – which take them quickly once they are palatable.

 

The seeds are extracted by macerating the fruits, fermenting the resultant material in warm water for a few days, rubbing them to disengage the flesh and then decanting off the dross, any floating seeds and the liquid. The extracted seeds (which will fall to the bottom of the container) should be thoroughly washed to clean them and are then surface dried.

 

Storage under water conserving conditions at cool temperature will maintain good viability for five or six years. The seed is protected by a bony seed coat which is hard and brittle but not impermeable to water – it is there chiefly to protect the seed in its passage through the gut of a bird and subsequently in the soil, protecting it against the ubiquitous rotting agencies while the various embryo maturation and dormancy breaking sequences are satisfied. These occur over a considerable time scale. Internally, at dispersal, the seed contains a minute, rudimentary embryo which is embedded in a copious, white, oily endosperm.

 

The embryo maturation process in Yews is very lengthy and, it would appear, cannot be hastened – merely raising the temperature above the required threshold does not significantly increase the rate of activity. There is little accurate information available on the time needed to mature the embryo (as traditionally the seeds were sown in open ground seedbeds and left to their own devices until a crop emerged) but provided that the seed is fully imbibed at the beginning of the treatment, then a period of 180 days at a temperature above 15ºC appears to be sufficient, importantly very warm temperatures should be avoided as the oily food reserve is prone to turn rancid.

 

Once the embryo has developed to its full size it then requires a period of chilling to overcome an endogenous dormancy – the periods quoted for this activity vary dramatically from a high figure of 120 days downwards to 60 days; the high figures suggest that the combinations practiced did not complete the maturation processes and that these were completed during the chilling period. As such an extreme for the duration of the chilling sequence does not occur in any other group of Gymnosperms, it seems unlikely that such a lengthy period is needed – the available evidence suggests that if the maturation phase has been fully completed then a period of the order of 56 days at below 3ºC should prove sufficient.

 

The treatment of Yew seeds is therefore a tedious and lengthy procedure, the time scale from collection – through extraction – pre-treatments both warm and cold – to sowing for germination will occupy a period of between eight and nine months. Thus as germination cannot definitely be achieved until the second spring after dispersal it would be prudent to allow for generous over runs in the warm and cold treatment phases in order to ensure that all encumbrances are completely removed. This will allow sowing to take place so that germination can occur early in the year as day length is extending and so provide a long growing season for seedling development. There is no particular benefit in paring down the treatment phases unless critical observations suggest a time scale for germination in the first spring is feasible.

 

Although there are many recommendations for the treatment of ‘naked’ seeds – the treatment of these roundish, bony seeds (c6mm in diameter) is best undertaken by extending in moist vermiculite in the prescribed fashion; this will at least reduce the possibility of ‘heating up’ during the warm phase – although this is unlikely to kill the seed it could upset the programme or cause rancidity of the food reserves.

 

Torreya

 

The ‘Plum Yews’ (Torreya) are a small genus of geographically local, evergreen bushes or small trees which closely resemble the Yews, however they are readily distinguished by their fruits which are relatively large – about 25mm in length.

 

The genus is usually recognised as containing six species and these are native to North America and North Eastern Asia in various warm temperate climates – T. californicum from the extreme South West of the USA; T. taxifolium from Florida, T. nucifera from Japan and T. grandis from China – are the most significant and all are remarkably similar to each other.

 

The plants produce their flowers in the spring and the fruits develop over two growing seasons and are ripe in the early part of the second autumn. The drupe-like arils have a fleshy – if somewhat dry – outer coat which dries out gradually turning from green to purple or brownish purple and then splits into a series of longitudinal fibres.

 

The seeds are extracted by rubbing off the fibres which generally come away quite easily to leave a beige coloured, egg shaped seed, about 25 mm in length, more or less flattened in one plane and pointed at both ends. The seed coat is thick and internally is folded and very irregular – encroaching into the endosperm. It is not impermeable to water. The seed contains a minute, rudimentary embryo in a thick, oily, white endosperm. Storage of the extracted seed is successful - under water conserving conditions at cool temperature - will give a longevity of several years.

 

The embryo is significantly immature and mirrors its close relatives, the Yews, in this respect. It will require a long period of warm storage of the imbibed seed to bring to a mature enough condition to move into the next phase of activity. It is probable that it will also require 180 days at above 15ºC – also avoiding the development of too high a temperature to avoid rancidity of the food reserves.

 

After embryo maturation, germination is further delayed by the need for a marginal period of chilling to break an endogenous dormancy. This can be achieved by stratifying the seeds for 28 days below 5ºC.

 

These Plum Yews are unusual in that the cotyledons remain trapped in the seed case at emergence and it is the first true leaves which appear from the germination substrate.

 

The remaining genera in this Family are little known in terms of their propagation from seed. They are unlikely to fruit in the UK and as such it will be necessary to import seed for the purpose. They will almost certainly have an immature embryo condition which will require maturation by warm stratification, Reference to their climatic provenance will determine if there is likely to be a need for chilling – albeit chilling may occur at a relatively high threshold temperature.

 

Successful growth of the seedlings of all species will eventually depend on the formation of a mycorrhizal association and this is of the vesicular-arbuscular type. It will be necessary to be aware of this and make whatever arrangements are relevant to the production process.