The classification of the Cycads is much discussed – as to whether there is one all embracing Family – the Cycadaceae – or the three families shown below. Currently over three hundred species, in eleven genera, are recognised - although it is believed there are many species as yet to be identified, as many species are very narrowly endemic and probably have not yet been discovered or recognised.

 

The Cycads

 

Family                             Genus                      Species                                 Distribution

 

Cycadaceae                   Cycas                       105                                       Old World

Stangeriaceae                Stangeria                      1                                       Southern Africa

                                       Bowenia                        2                                       Queensland

Zamiaceae                     Dioon                          13                                       Central America

                                       Encephalartos             65                                       South East Africa

                                       Macrozamia                 41                                       Australia

                                       Lepidozamia                  2                                       Australia

                                       Ceratozamia                26                                       Central America

                                       Microcycas                     1                                       Cuba

                                       Chigua                           2                                       Colombia

                                       Zamia                           65                                       New World

 

The Cycads are primitive, cone bearing, members of the Gymnospermae and antedate the conifers in evolutionary terms. The surviving examples are relics of a once dominant, world wide group of plants. Nowadays they are limited to specific and defined habitats around the world – principally in the tropics and sub-tropics but with some outliers at the fringes and which survive in warm temperate climates and can be grown in the mildest niches in the British Isles. They are chiefly plants of humid conditions but some – especially those with a temperate distribution are xerophytic. These interesting plants are rarely common even in their native habitats and in most cases they are becoming rare and endangered – as man despoils their environment or collects them and moves them to cultivation. The great majority of species are slow growing and can live to great age.

 

These higher, seed producing plants are unusual in that the seed is fully developed in size before pollination and fertilisation occurs. All the species are dioecious and pollination is by wind – although the process can be induced artificially.

 

Once the ovule has been fertilised further development is steady, continuous and inexorable – provided that the seed is not damaged. To this end the seed has a hard seed coat, which protects it and insulates it against the vagaries of its environment. Thus, despite many written recommendations to crack, scarify or hot water treat the seeds – they should be left as collected and extracted. The developing embryo does not need any additional water. The creation of a permeable seed coat, artificially, merely allows the possibility of drying with the consequent deterioration of the endosperm and/or the ingress of rotting agencies.

 

There is no quiescent stage – as in higher seed producing plants – and the embryo continues developing into the germination phase regardless of environmental conditions – it does not need the stimulation of warmth or moisture to achieve progress. Structurally the seed carries a large and extensive endosperm in which the embryo is enclosed. This provides support for the developing seedling for some considerable time – even after germination and the deployment of photosynthetically active leaves.

 

When the seed is shed from the cone it is drupaceous in structure; it is covered in a dryish, brightly coloured flesh which is attractive as a food source to various birds and animals. It is thus an effective agency in ensuring dispersal. The hard seed coat is therefore also a protection against abrasion by teeth, bills etc and then passage through the gut and the digestive processes.

 

At the moment of dispersal the embryo is undifferentiated but it will then continue to develop, often taking six to seven months to mature to a stage at which germination and radicle emergence will occur.

 

For crop production the cones are collected as they reach a stage at which the seeds can be separated from the cone scales. They are extracted from the flesh by soaking in warm water for a few days; this causes the flesh to soften and ferment. This degraded tissue is then rubbed free and the detritus and floating, non-viable seeds can be decanted off. The seeds are washed clean to remove any further adhering flesh (which may provide a substrate for potentially rotting organisms) and then surface dried for ease of handling.

 

As the seed coat is hard and the developmental processes are not sensitive to the environment – except perhaps low temperatures; no benefit accrues from sowing the seeds until just prior to the stage at which the seed coat cracks and the hypocotyledonary tube emerges.

 

The seeds are therefore stored after surface sterilisation (to prevent the development of rots), extended in some moist vermiculite at a temperature of c25˚C for the required period (usually several months) until the cracking is visible. Sometime prior to this it would be prudent to float the seeds again to separate any previously undetected non-viable material.

 

The cracking seed is sown by pressing it into the medium – usually sand, vermiculite, perlite etc – to leave about two thirds of the seed exposed. It is then subjected to a humid environment and warmth (25˚C+). If the ‘radicle’ (which is in fact a tube made up of two tightly adpressed cotyledons) has emerged it can be angled down towards the medium. The radicle proper will then emerge from the tube and can be recognised by its small, dry, brown tip. As the radicle enters the soil it swells to form a tuberous tap root with numerous lateral roots. After six weeks or so, when the tap root is well established, the first leaves emerge by splitting the cotyledonary tube. At this stage the seedlings are lifted and potted individually.

 

The cotyledons continue to derive nutrients from the endosperm until well after germination and this supplements the requirements of the seedling for a considerable period: thus the seed should not be separated from the seedling, as it may well remain in place and be useful for as much as two years.

 

If a warm environment is maintained, germination is remarkably synchronised. Inevitably a more accurate assessment of the storage time for maturation will be made if the seed is obtained as soon after the dispersal phase as possible.

 

In general Cycads are high value crops – largely because they take several years to reach a maturity at which a complete crown of leaves is developed. The juvenile leaves differ from the adult leaves, often quite significantly.

 

The seeds are in general scarce and/or expensive to buy in.