I believe that "pine, bamboo, and plum" are considerd the "three friends of winter"...You can look it up for more detail.......
"Fire and Water" within onesself refers, I believe, to the internal alchemy that needs to take place when they are combined in order to anchieve some level of enlightenment.
Could be wrong....
Regarding Morihei Ueshiba and his writing/sayings/doka/etc, there are - in general - two layers: spiritual and physical.
For the longest time, everyone was focusing on his sayings as entirely spiritual and everyone was lost. (Everyone meaning 99.9% I'm sure there were exceptions out there.) Ueshiba, himself, cleared up this dilemna by saying that you didn't have to follow in his spiritual footsteps. You could pick pretty much anything. Aiki made it better. Aiki being the martial concept.
Because of that and because we are now finding that Ueshiba was also referring to some fairly old physical, martial training concepts, we have recently begun to look at what Ueshiba was saying in reference to those martial training concepts.
Spiritually - someone else can dig into those meanings.
Martially - He meant specific training concepts. What were they?
For instance, he would write ka and mi (fire and water) as kami. Now, when he's speaking, did he mean kami as in spirits or kami as in fire/water? When someone wrote down what he said, did they get the right "kami"?
Refer to Chris Li's blogs for more examples.