Hello NeedAccountability, I'm sorry for taking a while to get back to you, I will do my best to answer your questions.

How do I determine where the light source is coming from?

In order to identify where the lightsource is you must first identify the type of shadow present in your Reference. Find a specific form that is present in your reference, a scale, a bump, any kind of small form present in the surface of your structure that casts a shadow, if the form is on top of your structure then your shadow will likely be cast on the other side, so your lightsource will be on the opposite side of the shadows.

But if your texture is made up of holes or cracks present on top of the surface then the light will actually hit the walls and the floor on the other side, and instead you should focus on the walls and floor, what their size is and the angle of the light, as this will dictate the shadow that is cast onto the other side of the crack.

I'd generally recommend that before you even start to add texture to your work that you hover your hand above your reference and try to "trace back" to where the lightsource is until you find it's general direction, then it's a matter of keeping this knowledge in mind when you add shadows to your work, so that all of them respect the lightsource and don't contradict it.

Remember the last column in the texture analysis exercise from Lesson 2? We're simply following the same principles introduced there and applying it to a tridimensional structure, rather than a flat surface.

Because we're working with a tridimensional structure this also means we must be doubly aware of the angle at which the light hits the form, as shown in this diagram.

How far away a form is from the light source will change the angle of the light rays that hit the object, for example light rays that hit the object at a shallower angle the farther away it is will result in the shadow itself being projected farther. We can use this to our advantage and use it to create focal points of detail, with certain areas being more prominently detailed, and others having less contrast - and as such they will draw less attention from the viewer.

The reason for this question is if I look at a plant reference where say sunlight or light is hitting it full on, how do I begin approaching drawing cast shadows when there are literally no shadows?

There is likely always some degree of shadows going to be present in your work unless your photo is being fully blasted straight on with several light sources that wash out all of the shadows in your reference picture, but this is a very specific and unlikely scenario, most of the pictures you'll find will likely contain some amount of shadows.

In general in order to get the most out of this exercise and further your understanding of how texture works you should look for good photos with a high definition and a clear lightsource and shadows, natural sunlight is highly recommended. Sometimes photos will have more than one lightsource which can create more shadows, eliminate them, or soften them out, so when looking for references try to choose photos with a clear, single lightsource.

2) This is sort of tied to the first question, but I have an easier time conceptualizing cast shadows when the surfaces are super spiky and have protrusions (reason being that they cast very obvious shadows). But when surfaces are a bit smoother (like the surface of a leaf) where there are very slight grooves and bumps, how do you determine where the shadow is cast?

This is why having a picture with a high quality is tantamount to capturing texture, photos of lower quality will flatten, wash out, and simplify the small textures present in the surfaces of forms too much, and make it difficult for you to analyze and understand them.

For example leaves are incredibly varied structures and a lot of them aren't smooth, they actually have several small veins along their surface, search for "leaf texture" on search engines such as Pinterest and you'll see that they actually have a lot of grooves and veins running along them, the shadows these small structures cast then convey the type of texture these objects posses, it is the same concept as any other type of texture, just on a smaller and thus less noticeable scale.

I recommend doing the 25 textures challenge if you wish to learn more about texture, there's nothing better than hands on practice, and you will also have the opportunity to recieve more feedback on your work that is specific to texture. Just keep in mind that this challenge is supposed to be completed in tandem with other lessons, and not finished in one go, this will help you develop your skills much more than cramming it all at once.