Question: I made a cold frame out of $5.00 worth of lumber and some saran wrap. I don't know if it necessarily deserves the title "cold frame", but I want to use it as a place that is protected from the elements where I can sprout and keep seedlings during the day. 

I know that one usually uses a cold frame to raise early lettuce and other things in, but is a cold frame an acceptable alternative for a greenhouse?

Answer: 
It's all genetic. Here is a list of the taxonomies of several common curcurbits:

Watermelon:

* **Kingdom:** Plantae
* **Phylum:** Magnoliophyta
* **Class:** Magnoliopsida
* **Order:** Violales
* **Family:** Curcurbitaceae
* **Genus:** Citrullus
* **Species:** *lanatus*

Squash:

* **Kingdom:** Plantae
* **Phylum:** Magnoliophyta
* **Class::** Magnoliopsida
* **Order:** Violales
* **Family:** Curcurbitaceae
* **Genus:** Curcurbita
* **Species:** Pumpkin: *maxima*, Butternut type: *moschata*, Summer type: *pepo*

Cucumber + Melon:

* **Kingdom:** Plantae
* **Phylum:** Magnoliophyta
* **Class:** Magnoliopsida
* **Order:** Violales
* **Family:** Curcurbitaceae
* **Genus:** Cucumis
* **Species:** Cucumber: *sativus*, Melon: *melo*

As you can see, the watermelon, squash, and melon are in the same family, But each has it's own Genus. Of course, every species of plant has its own set of characteristics. Watermelon vines are thinner and longer than squash vines, and the leaves are deeply lobed, compared with the squashes wide, flat leaves. 

The fruit looks very similar when young, but when they are matured the watermelons have a dramatic decrease in starch content, and have low density flesh with extremely high moisture content, and a high level of fructose. The cucumber genus is lower in water content, but also store the energy supply as fructose rather than starch. The melons have more sugar than the cucumbers. 

Notice that the ones with the higher starch ripen latest in the season, when animals need starch to build up fat for winter. The more sugary juicier fruits are ripened a little earlier, when the animals need quick energy.
The above answer is accurate. The simple gist of it is this:

There are two sensation categories involved: Flavor and Sweetness

There are two substance categories involved: Starch and Water

Listed below are the three fruits in question, by each of the above four items in the order of their volume from most to least:

1. **Cucumber**: water, starch, sugar, flavor.
2. **Squash**: starch, flavor, or flavor, starch, sugar, water.
3. **Melon**: Flavor, water, or water, flavor, sugar but no starch