Giovanni Battista Amici (1824) who was also a good microscope maker, found that the stigma of Portulaca oleracea was covered with hairs which contained some granules or particles inside them. Curiosity prompted him to ascertain whether they moved in the same way as the granules he had seen in the cells of Chara. It pleased him to find that they did.
While repeating the observation, he accidentally saw a pollen grain attached to the hair he had under observation. Suddenly the pollen grain split open and sent out a kind of tube or “gut” which grew along the side of the hair and entered the tissues of the stigma. For three hours he kept it under observation and watched the cytoplasmic granules circulate inside it, but eventually he lost sight of them and could not say whether they returned to the grain, entered the stigma, or dissolved away in some manner.
Amici’s discovery stimulated the young French botanist Brongniart (1827) to examine a large number of pollinated pistils with a view to understanding the interaction between the pollen and the stigma and the introduction of the fertilizing substance into the ovule. He found the formation of the pollen tubes (he called them spermatic tubules) to be a very frequent occurrence but persuaded himself to believe that, after penetrating the stigma, the tubes burst and discharged their granular contents, which he likened to the spermatozoids of animals and considered to be the active part of the pollen. He thought he saw these “spermatic granules” vibrating down the whole length of the style and entering the placenta and ovule, and he drew a series of figures to illustrate the whole process.
In appreciation of this work, Brongniart was awarded a prize by the Paris Academy of Sciences and recommended for admission to the Academy.
Amici (1830) applied himself once again to the problem, studying Portulaca oleracea, Hibiscus syriacus, and other plants, and wrote a letter to Mirbel in which he put the following question:
“Is the prolific humor (means liquid containing tube) passed out into the interstices (means inside gap) of the transmitting tissue of the style, as Brongniart has seen and drawn it, 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?”
His observations completely ruled out the first alternative, and he definitely concluded in favor of the second.
About the same time, Robert Brown (1831, 1833) saw pollen grains on the stigmas and pollen tubes in the ovaries of certain orchids and asclepiads (members of Asclepiadaceae) but was uncertain as to whether the tubes were always connected with the pollen grains. He thought instead that, at least in some cases, the tubes arose within the style itself, although possibly they were stimulated to develop in consequence of the pollination of the stigma.
Source: Introduction to Embryology of Angiosperm by P. Maheshwari.