In 1999, NASA flew Ceratopteris fern spores on STS-93 to test gravity-guided growth; they germinated normally, but their developmental polarity became random

All organisms react to external stimuli. Roots develop downwards and shoots develop upwards. Even the small spore of a fern, which cannot be seen without a microscope, is able to develop itself in the right direction when it starts growing. The directional ability is partially based on gravity.

In 1999, an experiment was carried out by NASA in which the fern Ceratopteris richardii was sent up in space on the Space Shuttle mission STS-93. In the experiment, NASA wanted to study what would happen when gravity was no longer present as a directional clue. The germination process took place in the space shuttle under the necessary conditions for development. The only exception was the absence of gravity, where the spores could develop without having the usual gravitational cue for direction.

The importance of direction in plant development



This clearly explains how important directional clues are for plants during their growth process. There is much that happens every day depending on these small clues that most of us usually ignore. Some of the environmental stimuli that a growing plant responds to include gravity.

On Earth, a developing fern normally receives a consistent gravitational cue that helps establish which way its tissues should grow. When that cue was absent during the STS-93 experiment, the spores did not simply stop developing. Instead, they continued through development, but their normal directional pattern became less predictable.

The experiment therefore helped distinguish two processes that normally occur together: development and orientation. The spores were able to germinate and continue developing in the spaceflight environment, while the absence of gravity affected the direction in which that development was organized. This allowed scientists to examine the role of an environmental stimulus without removing the basic conditions required for germination.

Small environmental clues can guide growth

Under normal Earth conditions, plant orientation is constantly affected by gravity without any action on the part of the organism. The constant stimulus of gravity provides the growing plant with an environmental clue for setting its orientation. Without this familiar cue, the organism no longer received the same information for establishing its direction, while other aspects of the environment remained favorable for growth. The Ceratopteris experiment showed this effect at a fundamental level. Removing gravity did not prevent the biological process from taking place; it altered the spatial organization of that process.

This is why the experiment mattered. Growth is not controlled by a single factor. An organism can have the water, temperature, nutrients, and other conditions it needs to develop, yet environmental signals can still influence how that development is organized. Gravity is one such signal, and on Earth its constant presence means its role can easily go unnoticed.

The spaceflight experiment temporarily removed this familiar reference for a few days. In doing so, it allowed scientists to observe more precisely what gravity contributes to the growth of plants. The spores showed that germination and development could continue in the absence of the usual gravitational cue, while the establishment of a consistent growth direction was affected.

Bringing the lesson home

This does not mean that gravity is the sole influence on plant growth. Instead, it indicates how an unchanging environmental stimulus can serve as an important element in a biological process. Plants grow in conditions and environments on Earth that include gravity, along with many other physical and chemical cues.

The experiment also illustrates why seemingly ordinary environmental conditions can be scientifically important. Gravity is always present on Earth, so its specific contribution to plant development can be difficult to isolate under normal conditions. Only when the spores were placed in the microgravity environment of STS-93 could scientists observe what happened when that familiar cue was removed.

The result was straightforward: the spores continued to develop, but their growth no longer followed the same predictable directional pattern. The experiment therefore showed that development could continue without the usual gravitational reference, while the orientation of that development could change.