We favor halogen lights because, like other common types of indoor lighting, they provide an acceptable light qualitya white light akin to sunlight, in which colors appear natural and normal to our eyes. Halogen light is likewise suitable for plants, which can photosynthesize and grow without sunlight as long as they are given something similar.
The plants we select to grow indoors, where the quantity of light is often low, are typically native to habitats that have little available sunlight, and where temperatures are reasonably high and constant throughout the year. Not surprisingly, then, a large number of popular indoor plants are native to the tropics. The various Philodendron species, for example, come from places like Brazil and Colombia.
Image: UNIVERSITY OF PENNSYLVANIA SCHOOL OF VETERINARY MEDICINE
Philodendron means "tree-loving," and most species in this genus are tree-climbing vines. Vines seek light by using trees to reach the top of what ecologists call the "canopy," where sunlight is available, unobscured by taller plants. Thus if your Philodendron is given free rein, you can expect it will grow across the room toward your halogen light.
Whether we consider a Philodendron growing indoors or one in the forest, a deeper question follows: How does this plant survive at a very low light level until it reaches the sunlight, or halogen lamp? To answer we need to understand how plants absorb light to make sugars and starchthat is, photosynthesize. Chlorophyll is the familiar pigment that gives plants their green color, but there is no free chlorophyll in the leafit is all bundled with specific proteins. The most abundant of these packages, the light-harvesting protein complex, absorbs light energy and passes it on to other complexes that begin the actual photosynthetic process. When a plant is short of light, it makes more of this light-harvesting complex; when it has plenty of light, it makes less. As a result, leaves at the ground level of a vine growing in the forest are a deeper green than leaves high up on the vine. Leaves are thus adapted to the light intensities they experience.
Indoor plant owners should bear this adaptation in mind. Because it requires protein synthesis and turnover, adaptation to light intensity takes several days or weeks; so you should not take a house plant from indoor shade and give it sunlight by way of a treat, as it will get sunburned, turn brown and probably die. Indoor orchids are very susceptible in this regard.
There is another interesting consideration that stems from the initial question. The range of light intensities over which plants can adapt is highly variable between species and genera, and is genetically determined. Whereas Philodendron, by virtue of its tendency to climb, needs to adapt over a wide range of light intensities, a field plant typically does not. Thus we can grow the former successfully in dim light, but never a sunflower.