That is an interesting question. Most leaves reflect mid and far infrared radiation because of their high water content and cellular structure. However, some leaves have adaptations that allow them to transmit or absorb more infrared radiation, such as hairs, waxes, or pigments. For example, some desert plants have hairy leaves that reduce the reflection of infrared radiation and increase the emission of heat, which helps them cool down in hot environments. Some succulent plants have waxy coatings that increase the reflection of infrared radiation and reduce the water loss through transpiration. Some evergreen plants have pigments that absorb infrared radiation and protect them from photodamage in cold and dry conditions.

There is a region of the electromagnetic spectrum called the infrared window, which is a range of wavelengths from about 8 to 14 μm that can pass through the atmosphere with little absorption or scattering. This region is important for remote sensing applications, such as thermal imaging or weather satellites. However, not all leaves are transparent to this region of the spectrum. Some leaves have a higher emissivity than others, which means they emit more infrared radiation than they reflect. Emissivity depends on factors such as leaf surface characteristics, leaf angle, leaf temperature, and leaf water content. Generally, broad-leaved plants have higher emissivity than needle-leaved plants, and healthy plants have higher emissivity than stressed or diseased plants.

Based on this information, it seems that there is no single type of leaf that is completely transparent to mid infrared frequencies, but rather there is a variation in how different leaves interact with infrared radiation depending on their adaptations and environmental conditions.