b. The increasing humidity causes reduced transpiration rates. This is because atmosphere is saturated with water vapor and so not much can be absorbed by the plant as humidity increases. At 100 percent humidity, the plant cannot absorb any more water. In a dryer atmosphere (less humidity), transpiration will happen much more rapidly.
c. The rate of transpiration is related to whether the stomata are opened or closed. The curve levels off because something is limiting. Transpiration could be limited by humidity because water is evaporated much more slowly into an atmosphere that already has a lot of water vapor.
d. Anacharis lives in an environment where it doesn’t need to do transpiration (such as underwater), since it has no stomata. Stomata are for gas exchange, and Anacharis lives in an environment where gas exchange is done through diffusion in water. Water lilies live in a wet environment where it is only exposed to air on the top of its leaves, so that is where stomata are present and where gas exchange occurs. The bottoms of the leaves are submerged in water, so there is no gas exchanged there. The top of black walnut leaves are exposed to direct sunlight. However, the bottoms of the leaves are not because they are not directly exposed to the sunlight. To prevent water loss and to allow water exchange, the stomata are present on the lower epidermis.