Sex and the Single Cryptogam j 471471 471
animals, most diffentiated plant cells are potentially totipotent— that is, they can regenerate
a complete organism under the appropriate conditions. In other words, the somatic cells
of plants have nearly the same potential to generate a complete plant as the fertilized egg.
Viewed in this context, it is easy to understand why Schleiden concluded that plant embryo-
genesis is a vegetative process. Like Goethe, Schleiden regarded ovules as specialized vegeta-
tive buds, referring to them as “seed- buds.” Schleiden felt so confidant in his interpretation
that he proposed that the word “sex” be banished from the botanical literature:It will of course be understood that the word “sex” means nothing beyond a mere
indication, it being at any rate at present incorrect to attach to the term the meaning
current with respect to animal life. It would be highly desirable wholly to banish the
use of this equivocal term, as many misconceptions might be avoided. ^19Schleiden thought it preposterous that anyone would think plants capable of sex: “I am
surprised,” he scoffed, “that no one has yet insisted upon the presence of the organs of sense,
as eyes and ears, in plants, since they are possessed by animals.”
Schleiden also rejected the “enigmatical” distinction between cryptogams and seed
plants established by Linnaeus. Since he believed that cryptogam spores and the seeds of
seed plants were equivalent structures, he referred to them both as “spores.” Accordingly, he
devised a new system of classification that left out sexuality. He divided the plant kingdom
into two main groups: those with “covered spores” (Angiosporae) and those with naked
spores (Gymnosporae). He placed algae and fungi in the Angiosporae. The cryptogams,
on the other hand, he placed in the Gymnosporae under the heading Plantae agamicae—
plants in which the “spore” develops into a new plant directly. Seed plants, including flow-
ering plants, were all placed among the Gymnosporae as well, and were designated Planta
gamicae. In this group, the “spore” (in this case referring to pollen), like the spores of the
Plantae agamicae, develop asexually into a new plant but only by interacting with special-
ized cells in the ovule of the parent plant.
To understand Schleiden’s asexual theory of embryogenesis, we will first need a brief
introduction to the pollen tube.Amici’s Discovery of the Pollen Tube
Following Nehemiah Grew’s proposal of the sexual role of pollen in 1682, nearly 150 years
were to elapse before the pollen tube was discovered. Thus, for more than a century, the
process by which pollen grains landing on the stigma managed to fertilize the ovule con-
tained in the ovary was a black box. Grew had speculated that the pollen grain spilled onto
the outer surface of the ovary, “touching it with a prolific virtue.” Koelreuter and Sprengel
had shown that pollen must land on the stigma to effect fertilization. Samuel Morland in
England, Geoffroy in France, and Malpighi in Italy advanced the theory that entire pollen
grains slid down a central cavity in hollow styles to reach the ovule— but relatively few flow-
ers have hollow styles. Several other observers, including Linnaeus, had suggested that pol-
len grains burst open on the stigma and the released contents diffused down the style into
the ovule. Koelreuter, who opposed the idea that the pollen burst on the stigma, suggested