Cones & Seeds
Cycad plants superficially resemble palms or tree ferns but differ greatly in details of their structure and reproductive behaviour. All cycad species bear male and female reproductive structures on separate plants and reproduction is by seeds that are produced on modified leaves known as sporophylls.
Newly ripened cycad seeds are usually composed of four parts. The integument has three layers:
- an outer fleshy layer called the sarcotesta
- a central stony layer known as the sclerotesta (or shell)
- an inner membrane called the endotesta.
These 3 layers surround and protect the central kernel, which contains the female gametophyte or megapametophyte, eventually the embryo.
Cycad seeds are produced in a wide range of size, shape, and surface texture. The largest seeds are probably those of Macrozamia macdonnellii or Cycas micronesica, and the smallest, Zamia pygmaea. The shape of most cycad seeds is generally ovoid, egg shaped, but they may also be globular or cylindrical.
The fertilized embryo develops slowly but continuously until germination, with short-term chemical inhibition of germination by the sarcotesta but no real period of dormancy. This lack of dormancy makes seeds relatively short-lived and prone to damage by desiccation.
The fleshy outer seedcoat attracts birds, rodents, small marsupials and other animals as dispersal agents. In most cases, these animals eat off the fleshy coat but do not consume the whole seed. Seed dispersal is limited to the usually short distances that the animals carry the seed.
As the male cone matures, there is rapid elongation of the cone axis, causing a marked increase in length. The sporophylls separate, exposing the sporangia. Shortly after this elongation takes places, and possibly as a result of drying, the sporangia crack open, and the microspores (pollen grains) are shed onto the smooth upper surface of the sporophylls directly below. The ripening and rupturing of these sporangia follow a pattern starting at the base of the cone and progressing upward. The sporangia on each sporophyll ripen from the back (those closest to the cone axis) to the front.
Ceratozamia latifolia male cone
Female Cones - The life of a female cycad cone is much longer than that of a male. Both sexes have a similar period of development up until pollination, but after that event life of the male cone is finished and it simply decays. The female cone on the other hand continues development, nurturing the expanding ovules until they are seeds of sufficient maturity to be released.
After pollination the female cone of a cycad enlarges and becomes a hormone sink, resulting in the transport of large levels of high-energy compounds and nutrients into the developing ovules. The increase in size of the cones and ovules is initially rapid then slows down. Considerable stress is placed on the female plant during this development period and if moisture and nutrient levels are insufficient, then the leaves of the female plant may suffer and often drop prematurely and no new leaves are produced that year.
These take varying periods to mature their seeds depending on the species. Some species of Encephalartos may be as quick as six months whereas others such as Dioon edule and D. spinulosum may take from 12 – 16 months and Encephalartos transvenosus takes up to 18 months. Species of Macrozamia commonly take 6 – 8 months. Strangely cycads with the smallest cones, such as Zamia pumila may still require 9 – 12 months for their cones to mature.
At maturity the female cones begin to disintegrate from the apex downwards. Usually at this time the sporophylls fall off the central axis, often with the seeds still attached.
Pollination and Development of Fertile Seeds
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