The respiratory membrane is formed where the walls of a capillary touch the walls of an alveolus. At the respiratory membrane, gas exchange occurs freely between the air and blood through the extremely thin walls of the alveolus and capillary. Septal cells and macrophages are also found inside the alveoli.
Little or no pollen should be showing oil the actual flowers (those red or green button-like parts in the center of the colorful bracts). Be sure the plant is well wrapped when you take it outside on your trip home because exposure to low temperatures for even a short time can injure leaves and bracts. Unwrap the plant as soon as possible because the petioles (stems of the leaves and bracts) can droop and twist if the plant is left wrapped for too long. For maximum plant life, place your poinsettia near a sunny window Or Some other well-lighted areas Do not let any part of the plant touch cold window panes.
There'll be outlier trees-- I've seen sick trees drop their leaves in August, and leafed deciduous trees in January as far north as Manhattan-- but there's a clear peak. In tropical climates, there's more variation in when the trees drop their leaves: it has to do with wet and dry seasons, soil quality, wind and elevation. To get a view that looks like autumn in Mexico, for example, you have to go into the mountains.