We planted on Tuesday, and now it's Sunday, 5 days later. True to the seed packets, when I went out to check my garden this morning, there were squash cotyledons emerging from about 14 of my 58 squash hills. Good start!
So, for 5 days, the soil appeared barren, and then all of a sudden these fleshy green leaves have emerged. What's been happening down there under the soil?
I don't know much about seed physiology, but I googled the topic and found the first page of a 1956 review of the topic. While I'm sure that much has been learned since then, the authors describe the basic process very well. They write "A seed contains an embryonic plant in an inactive condition, and germination is the resumption of growth." Right there is a bit of a surprise. A resumption of growth. Because the growth started while the seed was forming in the fertilized ovary - part of the original flower.
"The young plant, protected by varying layers of living and dead tissue, has reserves for metabolism. In some seeds development takes place as soon as water is absorbed, but in others germination does not take place until additional requirements are met." Squash seeds are large, so they have lots of reserves for metabolism. They are the type of seeds that start to develop as soon as water is absorbed. They don't require scarification or cold treatment, for example.
"Three distinct stages are evident in germinating seeds, namely (a) imbibition of water, (b) cell elongation, and (c) increase in cell number. In a physiological sense, the start of germination depends on a coupling of respiration to growth. The established seedling results from resumption of development and its continuation through growth." So my seeds have been taking up water, increasing the size of their dried up cells, and multiplying their cells.
That's as far as I could read the article without a subscription to Annual Review of Plant Physiology.
A bulletin from Washington State University explains a little more: "The first sign of germination is the absorption of water -- lots of water. This activates an enzyme, respiration increases and plant cells are duplicated. Soon the embryo becomes too large, the seed coat bursts open and the growing plant emerges. The tip of the root is the first thing to emerge and it's first for good reason. It will anchor the seed in place, and allow the embryo to absorb water and nutrients from the surrounding soil."
So, tomorrow we'll give them some more water, and the rest of the seedlings should be up in a few days, respiring, photosynthesizing, and multiplying their cells into a young seedling.