Cacti from Seeds
It is now early November and the older cacti are now in their winter sleep. The younger cacti in the greenhouse will try to keep awake as late as possible; some stay awake into December. Fall is a good growing time for young cacti. The cooler nights allow for more storing of carbon dioxide after the seedling's stomates open. The carbon dioxide is chemically absorbed at night by forming a complex structure with a molecule that does more absorbing when its cool. (In chemistry-speak, the equilibrium constant for the absorption of carbon dioxide decreases as temperature increases, and, of necessity, increases as temperature decreases.) The carbon dioxide is then released during the much warmer day by the complex and made available to the mechanism by which cacti produce sugars and oxygen. This is just what a normal plant does, but a normal plant doesn't open its stomates at night. Normal plants open their stomates during the day, letting in as much carbon dioxide for carbon fixation as possible while the sun is shining. All plants do this fixation of carbon, called photosynthesis. That's how they make the sugar that is their food. Cacti and other succulents have evolved to open their stomates at night when there is less water loss through the stomates. Lower temperature results in less water loss. The vast majority of plants don't grow in the arid habitats of cacti (and other succulents), so water loss isn't such a big deal to them. I think the reason the larger cacti don't do that much fall growing is that they are big enough/old enough for reproduction, which is the whole point of existence. They would just as well get a good winter sleep to rest up for the energy loss involved in reproduction during the coming year. The smaller plants are not yet of flowering age (= reproduction) and so are packing on as much size as possible so that they also will get to sleep longer over the winter in a future season, because they will be older and bigger plants, ready for reproduction.
I'm not sure why I just wrote this story. I just created it because maybe someone would find it interesting. Can't help it. It's my 30 years of teaching chemistry talking.
It happens that I was thinking last winter about doing a demonstration on growing cacti from seed for the planned 2020 Labor Day cactus show at the Albuquerque Botanical Garden by my cactus club (The Cactus and Succulent Society of New Mexico). I decided to plant some number of seeds of four different species every two weeks or so, so that people could see growth rates of cacti in real time. That is, a 32 cell tray would have 8 units of 4 species, where each unit is a snapshot about two weeks apart. I fiddled with his idea, trying various species and presentations during the winter, and started the planting sequence in May. Of course, by that time the coronavirus problem was raging, and I didn't think we'd have the Labor Day show (we didn't), but I thought I'd do the demo for myself as a dry run for next year. As I got into it, I thought maybe some of you out there might like to see this beginning-of-cacti-from-seed show, and here it is. I'm somewhat disappointed in my photographs, probably because I can see the real thing in 3 dimensions in front of me, and a photo is of necessity flat, but maybe you'll enjoy the pictures anyway. You won't have the real cacti at your elbow to distract you as I do.
Each picture has the original sowing date as a title. The four species and the number of seeds sown of each species are: top left: Peniocereus greggii SNL1 50 seeds; bottom left: Echinocereus triglochidiatus "White Sands" 150 seeds; top right: Astrophytum capricorne 100 seeds; bottom right: Escobaria missouriensis caespitosa SNL155 150 seeds. The tray was kept under a bank of six 4-foot long LED lights (3 x 2 tube fixtures), on for 18 hr, in my house. The temperature rose to 80F - 85F during the day and cooled off to around 70F at night. The species chosen have been reliable germinators for me over the years. The number of seeds was chosen to get a good filling of the cells with plants for the show. This did usually happen, with a few exceptions. I'll comment on the numbers of seedlings as I write more under each picture. I will also give a count for the day I originally wrote this article, September 19. I should have done this final counting on Labor Day, of course, but it slipped my attention. (The reason my attention slipped - it happens that my medical adventures are continuing.)
Ok, look at the pictures. They were taken Labor Day September 7, 2020.
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