Science

Making the Invisible Visible

Through patient observation and carefully designed experiments, Nathanael Pringsheim transformed microscopic structures into evidence. His work helped clarify how cells arise, how sexual reproduction operates, and how living structures develop through time.

Cell Theory · Reproduction · Development

01 — Cell Theory

Cells from Cells

In the mid-nineteenth century, scientists already knew that plants and animals are made of cells. Matthias Schleiden and Theodor Schwann had made this idea the foundation of cell theory. But one fundamental question was still unresolved: Where do new cells come from?

Pringsheim found the answer by watching living plant cells under the microscope. His observations showed that new cells do not appear spontaneously. They arise when existing cells divide.

In the green alga Cladophora glomerata, Pringsheim followed this process step by step. His drawings show a new cell wall forming through the cytoplasm — the living material inside the cell — between the chloroplasts, the green structures that capture light for photosynthesis. It was direct visual evidence that a new cell arises through the division of an existing one.

The principle was later famously expressed by Rudolf Virchow as Omnis cellula e cellula: every cell comes from a cell. It became one of the foundations of modern biology.

Three successive stages of cell division in Cladophora glomerata — a new cell wall · b chloroplasts · c cytoplasm

*Cladophora glomerata*— successive stages of cell division.

Only the region where a new cell wall (a) is formed is shown.

02 — Reproduction

Fertilization & Life Cycles

In the 1850s, one of the great questions in botany was whether algae reproduced sexually at all. Scientists had seen different kinds of reproductive cells, but no one had actually watched fertilization take place.

Three stages of fertilization in Vaucheria sessilis — A antheridium · O oogonium · E egg cell · Z zygote

*Vaucheria sessilis*— before, during and after fertilization.

Using the green alga Vaucheria sessilis, Pringsheim observed fertilization directly under the microscope. The antheridium, the male reproductive organ, produces the sperm cells; the oogonium, the female reproductive organ, contains the egg.

Pringsheim watched the sperm cells reach the egg and followed what happened before, during and after fertilization. For the first time, sexual reproduction in an alga could be understood as a visible biological process rather than something inferred from separate observations.

Sexual reproduction emerged as a fundamental principle shared across the plant and animal kingdoms.

Life cycle illustration of Coleochaete irregularis — *Coleochaete irregularis*— life cycle.

Pringsheim soon realized that fertilization was only part of the story. His studies of Coleochaete irregularis revealed a remarkable pattern: sexual and asexual generations alternate during the life cycle.

This insight became especially important because the life cycle of Coleochaete irregularis shows striking similarities to that of mosses and other early land plants. The humble green alga therefore offered clues to how the ancestors of land plants may have evolved from aquatic green algae.

03 — Development

Oogonium Formation

A microscope does more than reveal structures too small for the naked eye. In Pringsheim’s hands, it became a way of watching biological change as it happened.

His studies of Oedogonium sp. provide a particularly elegant example. Pringsheim followed the formation of the oogonium — the female reproductive structure containing the egg — through a sequence of developmental stages.

The drawings capture a transformation rather than a static object. A cell divides, a new wall forms, and one of the resulting cells gradually changes its shape until the characteristic oogonium emerges.

By observing the same process over time, Pringsheim could reconstruct how a biological structure develops, step by step. Today, following living cells through time is fundamental to developmental biology.

Successive stages of oogonium formation in Oedogonium — *Oedogonium* sp.— oogonium formation.

Pringsheim made invisible structures visible — and revealed the biological processes behind them.