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Gaertner’s primary contribution to botany rests not on his demonstration of sex in
plants, which had already been well- established, but on the sheer volume of his hybridiza-
tion studies. Darwin cited Gaertner thirty times in his brief section on “Hybridism” in the
first edition of Origin of Species (1859), and Mendel cited him seventeen times in his historic
paper, “Experiments on Plant Hybrids,” published in 1866. Mendel also singled Gaertner
out as a worthy predecessor:Numerous careful observers ... have devoted a part of their lives to [plant hybridiza-
tion] with tireless persistence. Gaertner, especially, in his work, Hybrid Production in
the Plant Kingdom, has recorded very estimable observations.^8Coming from someone of Mendel’s monumental stature, this is very high praise indeed!The Cryptogam Conundrum
There remained one last impediment to the universal acceptance of the sexual theory: the
apparent absence of sex in the group of plants Linnaeus had named the cryptogams. The
term “Cryptogamia,” which literally means “hidden marriage,” includes all the members
of the plant kingdom that do not produce seeds (the “nonseed plants”): algae, bryophytes
(mosses, liverworts, and hornworts), ferns, horsetails, and club mosses.^9 In this section, we’ll
briefly examine the discovery of sex in the cryptogams from a historical perspective. Such an
approach is essential to understand the genesis of the ideas that ultimately led to the Theory
of Alternation of Generations, which revealed the unity of the life cycles of the cryptogams
and the seed plants and changed forever the way we think about flowers.
The angiosperms and gymnosperms— which make up the vast majority of the plant
kingdom— are comprised of what Linnaeus called the Nuptiae Publicae Plantarum, or
plants with “public marriages.” Today, this group is referred to either as the phanerogams
(“visible marriages”) adopted around 1814, or the spermatophytes (“seed plants”), which
came into use around 1897.^10
Naming the nonseed plants “cryptogams” was an act of faith on Linnaeus’s part because
it assumed that this group of plants, like the seed plants, reproduced sexually, even though
the sexual structures had not yet been identified. For years, botanists had searched for sexual
structures in the cryptogams without success. Linnaeus’s struggle to understand the sexual
life cycle of mosses is a case in point. He had observed that in the spring the leafy structures
of mosses, which he equated with the leafy stems of flowering plants, instead of producing
flowers, produced a small capsule at the end of a slender stalk, which, when mature, released
a fine powder. Today, we call the small capsule that sits atop the green, leafy stage of moss a
sporangium (literally, “spore vessel”) and the powder it contains, spores (Figure 17.1).
Linnaeus initially identified the moss capsule as an anther and the powder as pollen.
However, he soon changed his mind after observing that the powder of moss capsules behaved
nothing like the pollen from anthers. Instead of landing on a stigma, it fell to the ground,
and each powder particle germinated and formed another little moss plant (see Figure 17.1).
Although Linnaeus could not discern any of the microscopic intermediate structures that
formed during the transition of the spore to a leafy moss plant, the ability of the moss “pow-
der” to give rise to another moss plant suggested to Linnaeus that it must represent seeds.