The seed : morphology and physiology
A Cycad seed comprises different distinct layers : the embryo is located in the center and is surrounded by what is called the endotesta that constitutes the energy reserves that will feed the embryo and then the seedling for its early stafes. These reserves are enclosed in a protective hard shell called sclerotesta. This envelope is covered by a fleshy layer named sarcotesta, making it look like a berry. This layer is usually brightly colored (usually red but sometimes orange, yellow or white) , probably for animal dispersal, and of gelatinous or spongious nature. Germination inhibitors were found in the sarcotesta of various species, hence the need for cleaning prior to germination.These inhibitors may serve to prevent the seeds from germinating too close to the motherplant thus avoiding competition.
Cycad seeds are special in many ways regarding their development. In many species of angiosperms, seeds develop to reach a stationnary phase and wait for an external stimulus to sprout. But with cycads it’s a bit difference as the process of growth of the embryo is continuous, from pollination to germination. This explains why most cycad seeds can’t be stored for a long time and need to be sown when they’re ready to sprout.
The seed of different cycad species develop a different paces. A dehiscing cone (or strobilus) does not mean that the seeds are ready to sprout. It is thus necessary to have an idea of when the seeds were harvested and how long they’ll need to reach maturity in order to optimize chances of good germination. Usually observed maturity
Bowenia : 1 to 3 months
Ceratozamia : 3 to 6 months
Cycas : 4 to 12 months
Dioon : 0 to 2 months
Encephalartos : usually 6 months for south african species, less for tropical species
Lepidozamia : 3 months
Macrozamia : 3 months
Microcycas : 0 months, can’t be stored
Stangeria : 1 to 6 months
Zamia : 0 to 3 months.
These data work for seeds harvested at cone dehiscence (not before) and vary from one species to another.
When seeds were freshly harvested and are immature, it’s best to store them at room temperature and soak them every other week for 24 hours so that they remain hydrated. It’s also possible to keep the seeds in a slightly moist medium such as sphagnum or peat.
When buying seeds, it’s often difficult to know when they’ll be ready to sprout, thus it’s necessary to use the right germination methods.
It must be stressed that some cycads, such as Encephalartos, produce fully formed seeds in the absence of pollination. Such seeds are sterile and will never sprout.
If seeds are still covered by their sarcotesta, it’s necessary to clean them in order to avoid fungus development and suppress eventual germination inhibitors. Cleaning can be done using a knife to remove the fleshy layer that is more or less difficult to scrape depending of its texture. For more important quantities of seeds of gelatinous nature (Zamia for exemple), one can use and enzyme called pectinase. Available as a power, it mut be mixed in water and kept at the right temperature for optimal activity. The time needed for cleaning will vary according to temperature, water, seed and enzyme quantities.
When the seeds are clean, it’s useful to soak them for 24h. It rehydrates them which can be helpful when they’ve been stored dry for a while.
Some seeds might float in water. It doesn’t mean they’re unfertile. A seed floating vertically may mean that the embryo is pushing against the shell and formed an air bubble. If, after 24h of soaking, some seeds do not sink, it’s best to sow them apart from the others. Doing so, rotten seeds won’t contaminate the viable ones. At the opposite, sinking seeds do not equal fertile ones as unpollinated Encephalartos seeds will be full and heavy, thus sink but never sprout.
If you obtain a large quantity of seeds, it’s possible to determine their fertility and maturity by sacrificing a few. A longitudinal cut is done in order to observe the presence (or absence) of embryo and its stage of maturity. Beware, a very immature seed won’t have an embryo but the presence of a suspensor (a white coil) can indicate successful pollination.
Soaking seeds in a broad spectrum fungicide may help as it destroys spores present on the seeds.
Various methods can be used for sowing seeds. One must keep in mind that Cycad seeds successful germination relies on two main parameters: temperature (between 27°C and 32°C) and moisture (70%-100%). Seeds are better not sown directly into soil or compost as it makes it hard to monitor them and fungal infections are high.
The two main methos are :
Sowing on a mineral bed: A bed is made with perlite, sand, vermiculite or other mineral medium that is heated (with a heating cable for exemple). The bed is moist but not too much and seeds lay horizontally.It’s necessary to keep it humid enough to allow germination.
Sowing using the baggy method : This method is widely used as it allows homogeneous moisture around seeds. Only zip plastic bags and sphagnum (or peat) are necessary. These mediums are slightly acidic and bear antifungal properties. The sowing medium must be moist but not too much and can be sterilized using a microwave for 5 minutes at max power, allowed to cool down before use. The plastic bag is then filled with medium and seeds and kept in a warm spot. If the medium seem too humid, the bag can be left open for a few days until it reaches the required amount of dryness. In the case of sphagnum, no water should be released when pressed with hands.
The advantages of this method is humidity control, different batches of seeds separation and low space needed (compared to the bed).
Beware, if the medium is too damp, seeds might split by absorbing water and be more prone to fungal attacks. it’s even truer for immature seeds.
One can control germinations once or twice a week and when the sprout is around 0,5-3cm, sprouted seeds can be potted up.
– Always buy fresh seeds. Some seed sellers often offer old and non viable seeds because they’re unaware of their conservation requirements.
– When the seeds are in the germination medium, some may show mold or rot. It’s better to remove those seeds so that they don’t infect the healthy ones.
– Good quality seeds usually don’t rot even if sown a bit early. However, some species seem to bear fungus endophytes and even if surface treated, they may rot anyway.