I have never heard of a 25 hour cycle. I assume he was referring to the photosynthesis process, on which there are many straight-forward books.
Plants in general respond primarily to the amount of light in a given 24 hour period, reflecting seasonal trends. Buds swell and break according to the length of the day, not the temperature. Temperature would affect the rate of leaf growth. Similarly leaf colour change in the fall and increased root growth in September/October are triggered by decreasing amounts of daylight.
If he was referring to something other than light, please let us know.