Final answer:
Soybeans are not classified as C3 plants because they employ the C4 photosynthetic pathway, which is suited to high light and temperature conditions.
Step-by-step explanation:
The question asks which plants are not included in so-called C3 plants. C3 plants use the most common photosynthetic pathway, which is the Calvin cycle, and they are typically most efficient in cool, wet climates. The options provided include storage organs like roots and tubers, trees and bushes, cool-season grasses and grains, and soybeans. Of these, soybeans are not included in C3 plants because they use a different photosynthetic pathway known as C4 photosynthesis which is more efficient in high light and temperature conditions.