Pseudobulbs are structures that orchids use to store food, nutrients, air, and water. They are terminal stem/branch growths coming off the rhizome (which is the part of the plant that is parallel to the surface of the media). They photosynthesize along with the leaves, though they don’t photosynthesize nearly as much as the leaves do. Some orchids have pseudobulbs (e.g. Cattleya, Bulbophyllum, etc.), and others don’t (e.g. Phalaenopsis, Habenaria, Cochleanthes, etc.). Of the orchids that have pseudobulbs, there are two types – regular bulbous pseudobulbs, like you see in Cattleya or Bulbophyllum, or cane pseudobulbs like you see in Dendrobium. By definition, because pseudobulbs store water, they are technically succulent structures[1], which means that orchids that have them experience some kind of water shortage whether it’s from cooking dry in the direct sun at midday or having a deciduous dry season in its native environment.

Common wisdom holds that mature orchid pseudobulbs photosynthesize. They do, but to a significantly lesser extent than new growths. To demonstrate this, we used a shortwave UV light and have shone it on a cattleya. As you will see in the photo below, chlorophyll fluoresces red. The highest concentration of chlorophyll is clearly in the new growth of the orchid. The fact that orchids are slow to grow may be due in part to the reduction of chlorophyll production as the growth matures on the orchid. It is not clear why orchids do this.

Figure 1 - Cattleya White Diamond with new growth under UV light.  Chlorophyll fluoresces red, and you can see here that most of the photosynthesis occurs in the new growth, shown in the center. The pseudobulbs, while they contain a little bit of chlorophyll, are not the main source of photosynthesis of the plant like the new growth is.  Photo by Christopher Satch

Pseudobulbs and Photosynthesis

Although pseudobulbs have less chlorophyll than the leaves, and even less than new growths, they do still take an active part in photosynthesis (the process by which plants feed themselves).

Figure 2 - Pseudobulbs help store carbon dioxide at night for photosynthesis during the day.  This allows the orchid to close its stomatal pores when it gets too dry but still allows it to photosynthesize. 

https://www.researchgate.net/publication/236338039 Spatial patterns of photosynthesis in thin-and thick-leaved epiphytic orchids: Unravelling C3-CAM plasticity in an organ-compartmented way Article  in  Annals of Botany · April 2013 DOI: 10.1093/aob/mct090 · Source: PubMed

As shown in the above figure, orchids use the pseudobulb to store carbon dioxide in large ducts called aerenchyma within the pseudobulb.  This allows for a steady supply of carbon dioxide for photosynthesis even if the orchid closes its pores.  An orchid may close its pores during the day to save water from dry environments.  This allows it to be efficient at photosynthesis while saving water.  This special form of photosynthesis that most pseudobulbous orchids do is called CAM photosynthesis.

Cane Pseudobulbs

Canes are branches of the rhizome that can branch again.  When a cane branches again, it creates a keiki (baby orchid) that is hanging off the cane.  Canes are technically indeterminate growers, as they can grow keikis on keikis, and continue to keep branching in that way.  Though, each cane is determinate – there is a terminus to each one’s growth.  It is thought that because keikis also have roots, they are a way for long-caned orchids to “tree-hop” and establish themselves vegetatively on neighboring trees.

Dendrobium are the most common cane orchids that you will run into.  Often, growers wonder why their Dendrobium will keiki rather than bloom.  This has to do with the plant receiving the right signal at the right time.  Most Dendrobium come from seasonally tropical areas – typically with a wet/dry season or a hot/cool season, or both.  Either way, the weather becomes unfavorable.  Dendrobium have an internal clock for blooming that expects the seasonal change at a certain time of the year.  When this seasonal change happens at the right time, Dendrobium will drop their leaves and bloom (or, for nondeciduous Dendrobium, they will just bloom).  However, if the Dendrobium is watered too much or kept in conditions that are favorable and conducive to growth, then the internal clock will not receive the external cues, the plant will think that summer has not ended, and the plant will keiki.

Keikis

Keikis are baby orchids that grow off of either a flower stalk or a cane of an orchid.  The most common orchids that keiki are Dendrobium, Catasetiinae, and Phalaenopsis.  Keikis are a way in which orchids clone themselves (asexual reproduction).

The word keiki (pronounced “KAY-kee”) is Hawaiian for “little one” or “child”, and indeed, keikis are baby orchids.  Orchids which keiki have evolved in environments that experience seasonal wet-dry cycles.  When an orchid keikis, it’s not a bad thing – it just means that the conditions for flowering have not been met.  You may hear old-time growers refer to keiki-ing as the result of you “being too nice” to your orchids.  In a way, they are right.  Plants that keiki (esp. Dendrobium) are very attuned to environmental conditions, and they take flowering cues from their environment, which is usually drought, cooling off, or both.  Plants that keiki have internal clocks that expect seasonal change at specific times of the year. If these conditions are not met in the time window in which they are expected, then the orchid thinks that it’s still the growing season (summer), and chooses to branch its canes (keiki) instead of flowering.

That behavior is typical for many plants – when growing conditions are favorable for vegetative growth, the plant chooses to work on itself and grow more foliage to become bigger and stronger.  When the orchid senses an environmental change, it switches to gearing up for sexual reproduction.  In changing environments, it is a better strategy to shuffle the genes by sexually reproducing, to create offspring that may (or may not) be better adapted to survive.  Nature doesn't waste energy and flowering and subsequent seed production is energy intensive. In stable, favorable environments where there is little driving force for seed reproduction, plant's may simply clone themselves instead.

The presence of keikis does not harm the mother plant if they are left attached. Keikis can be removed and potted or mounted when they have developed sufficiently to have leaves and roots. For phalaenopsis, four leaves and roots that are at least 1 3/4 inch (4 cm) long.

[1] Gotsch, S.G., Williams, C.B., Bicaba, R. et al. Trade-offs between succulent and non-succulent epiphytes underlie variation in drought tolerance and avoidance. Oecologia 198, 645–661 (2022). https://doi.org/10.1007/s00442-022-05140-9