How we cite our quotes:
And there the motor car stood, with drawn blinds, and upon them a curious pattern like a tree, Septimus thought, and this gradual drawing together of everything to one centre before his eyes, as if some horror had come almost to the surface and was about to burst into flames, terrified him. (1.33)
Even when looking at something as ordinary as a motor car, Septimus can become terrified. Everyday life is now just as frightening as his memories of war.
So, thought Septimus, looking up, they are signalling to me. Not indeed in actual words; that is, he could not read the language yet; but it was plain enough, this beauty, this exquisite beauty, and tears filled his eyes as he looked at the smoke words languishing and melting in the sky […]. (1.60)
Part of Septimus’ madness is striving for communication. He thinks birds and airplanes are trying to tell him something, and in the end, he attempts to communicate through his suicide.
The excitement of the elm trees rising and falling, rising and falling with all their leaves alight and the colour thinning and thickening from blue to the green of a hollow wave, like plumes on horses' heads, feathers on ladies', so proudly they rose and fell, so superbly, would have sent him mad. But he would not go mad. He would shut his eyes; he would see no more. (1.62)
Having experienced sheer terror, Septimus is really moved by visions of beauty (heck, we’re moved by beauty even without this terror business). The trees are very suggestive to him, just as flowers are suggestive to Clarissa.