keys to the vascular plants of east texas - Botanical Research ...

402 GINKGOPHYTA
abundance. Cycads were so numerous during the Mesozoic that this geologic period is sometimes
called the “Age of Cycads and Dinosaurs” (Raven et al. 1999). Cycads superficially resemble palms
(unbranched trunks, large pinnate leaves) and are sometimes referred to as “sago palms” or “Easter
palms,” but they are clearly unrelated to palms, as evidenced by numerous characteristics.
These include naked seeds borne in cones, multi-flagellated swimming sperm, and separate sexes
(plants dioecious). While we have no reports of them naturalizing in East TX, they are cultivated,
and this brief write up on the Cycadophyta is included for taxonomic interest and for use in college
botany classes. A number of cycads have been used as a source of human food—e.g., sago
starch from the starch-rich pith of the stems of several Cycas species. � The seeds of Cycas species
(Cycadaceae) have been used as a dietary source of polysaccharides for humans in various
western Pacific Islands. However, because they are known to be toxic, the seeds are cut and soaked
for an extended period with several changes of water. The toxicity of unprocessed seeds is due to
cycasin and macrozamin (glycosides which are water soluble and therefore removed by soaking),
which yield toxic molecules when hydrolyzed in the human intestine. These molecules and their
derivatives are reported to be hepatotoxic, carcinogenic, mutagenic, and teratogenic. On two Pacific
islands (Guam and New Guinea), a fatal degenerative neurological disease (symptoms: lateral
amyotrophic sclerosis, parkinsonism, dementia) has been attributed by some authorities to
other toxins in the flour made from Cycas seeds. These other toxins may include a neurotoxic
amino acid (L-BMAA). Further, ingestion of a variety of cycads is known to cause various symptoms
of poisoning (e.g., severe gastrointestinal tract disturbances, liver necrosis, neurologic symptoms—hind
limb paralysis in cattle, depression in dogs) in a variety of animal species as well as in
humans; one or two seeds have been reported to be sufficient to kill a dog (Whiting 1989;
Albretsen et al. 1998; Bruneton 1999; Burrows & Tyrl 2001). Since cycads are widely used as ornamentals,
care should be taken to limit access to the plants by children and animals.
REFERENCES: Read & Solt 1986; Whiting 1989; Stevenson 1990a, 1990b, 1992; Stevenson et al.
1990; Jones 2002; Whitelock 2002.
DIVISION GINKGOPHYTA
MAIDEN-HAIR TREES
AA division represented by a single surviving species, Ginkgo biloba L., GINKGO or MAIDEN-
HAIR TREE (Ginkgoaceae), native of China. Fossils almost identical to modern GINKGOS are
known from nearly 200 million years ago (Whetstone 1993), and the group was widespread
and abundant during the middle Mesozoic Era (Scagel et al. 1984; Mauseth 1998). Ginkgo is “often
referred to as a ‘living fossil’ because of its position as the sole surviving member of an ancient
lineage” (Scagel et al. 1984). This relict species is widely cultivated because of its resistance
to disease and pollution and its beautiful deciduous leaves which turn yellow in fall, but it is
“either extinct in the wild or drastically restricted in range” (Whetstone 1993). Page (1990a) reported
natural occurrence in remote valleys in China, but cultivated plants are descendants of
individuals grown in temple gardens of China and Japan (Moore et al. 1998). Ginkgo is distinctive
morphologically, having fan-shaped leaves (usually apically notched and hence the epithet
biloba) with open dichotomous venation. Individual trees can live to 1000 years old and reach
30 m in height, with trunks to 9 m in circumference (Scagel et al. 1984; Raven et al. 1999). It is
used as a street, yard, and ornamental tree in East TX. The species is dioecious, and care should
be taken to use staminate trees if possible, since ovulate trees produce abundant, stalked, naked
seeds the size of a small plum, which have a fleshy outer coat notorious for its rancid butterlike
foul-smell (due to butanoic and hexanoic acids—Raven et al. 1999). The edible seeds or
ginkgo nuts (minus the outer coat) are canned and sold in ethnic markets as “sal-nuts,” “silver
almonds,” or “white nuts” (Scagel et al. 1984; Whetstone 1993), and the scientific name is derived
from two Chinese words meaning “silver apricot” (Moore et al. 1998). � However, when