Flora Unveiled

472 i Flora Unveiled

that the pollen secreted an oily substance, the “essence” of the plant. He confessed igno-
rance as to what happened next, but he speculated that the male essence mixed with the
female essence on the stigmatic surface, and the blended essences then diffused down the
style to the ovule. Gaertner subscribed to a view similar to Kolreuter’s in his prize- winning
1849 treatise.
The solution to the problem of how the pollen grain delivers the fertilizing material to
the ovule had actually been discovered years earlier, in 1823, by Giovanni Battista Amici, a
Florentine mathematician, astronomer, and accomplished microscopist.^20 Amici observed
that the stigmas of Portulaca oleracea (common purslane) were covered with hairs contain-
ing swirling cytoplasm. Today we refer to such cytoplasmic swirling as “cytoplasmic stream-
ing.” While studying this phenomenon under the microscope, Amici happened to focus on
a single pollen grain attached to the stigmatic hair he was observing. To his astonishment, a
tubular “gut” suddenly emerged from the side of the pollen grain. For three hours he watched
as the tube grew down the side of the hair and disappeared into the tissues of the stigma.
Amici’s observations of pollen tubes were confirmed in 1827 by the French botanist
Adolphe- Théodore Brongniart. Brongniart assumed that after penetrating the stigma and
entering the style, the “spermatic tubules,” as he called them, burst open, releasing “sper-
matic granules,” which wriggled the rest of the way down to the ovule. However, Amici was
skeptical of Brongiart’s hypothesis. In 1830, after extending his own studies on Portulaca
and Hibiscus, Amici framed the question succinctly:

Is the prolific humor passed into the interstices of the transmitting tissue of the style ...

to be transported afterwards to the ovule, or is it that the pollen tubes elongate bit by

bit and finally come in contact with the ovules, one tube for each ovule?

Schleiden’s Asexual Female Parthenogenic Pollen Tubes

Schleiden was among the many microscopists who were inspired to study pollen tube devel-
opment after Amici’s and Brongniart’s reports. In 1837, he published a paper confirming
Amici’s observation that the pollen tube grew down through the style. He also reported
that he observed the pollen tube entering the ovule through a pore in the ovule’s surface.^21
Then, in 1838, Schleiden startled the scientific community with an astounding claim. He
reported that upon entering the ovule, the tip of the pollen tube divided to form a separate
cell, which then underwent repeated cell divisions inside the ovule to form the embryo.^22
In other words, the pollen tube is the sole source of the embryo that is present inside the
seed. The ovule serves only as an incubator that houses and nourishes the embryonic plant.
According to Schleiden, this pattern of pollen tube growth paralleled the pattern
observed during cryptogam spore development. For example, the germinating spores of
ferns initially grow as linear protonema filaments. After reaching a certain length they grow
laterally to form the flattened prothallus, which Schleiden regarded as the embryo precur-
sor, or “proembryo” (see Figure 17.3). Schleiden claimed that pollen grains followed the same
developmental program. Upon germinating on the stigma, they grow down the style to the
ovule by means of a protonema- like pollen tube. Upon entering the ovule, the tip of the
pollen tube then begins dividing laterally to form the embryo of the seed. The idea itself