photosynthetically

The story

1. Proto-Indo-European*bʰeh₂-
2. Proto-Indo-European*bʰéh₂os
3. Proto-Hellenic*pʰáwos
4. Ancient Greekφᾰ́ος (phắos)
5. Ancient Greekφῶς (phôs)
6. Ancient Greekφωτο- (phōto-)
7. Englishphoto-
8. Proto-Indo-European*ḱe?
9. Proto-Indo-European*ḱómder.?
10. Proto-Indo-European*sem-der.?
11. Proto-Hellenic*ksún
12. Ancient Greekσύν (sún)
13. Ancient Greekσυν- (sun-)
14. Proto-Indo-European*dʰeh₁-
15. Proto-Indo-European*dʰédʰeh₁ti
16. Ancient Greekτίθημι (títhēmi)
17. Ancient Greekσῠντῐ́θημῐ (sŭntĭ́thēmĭ)
18. Ancient Greekσύνθεσις (súnthesis)
19. Ancient Greekσυνθετικός (sunthetikós)
20. Frenchsynthétique
21. Englishsynthetic
22. Englishphotosynthetic
23. Proto-Indo-European*h₂el-
24. Proto-Indo-European*h₂élisder.?
25. Proto-Indo-European*leyg-
26. Proto-Germanic*līką
27. English todayphotosynthetically

From photosynthetic + -ally.