The spelling of the word "rectembryae" seems a bit odd, but it can be explained through its phonetic transcription: /rɛkˈtɛmbriaɪ/. The "r" sound is followed by a "ɛ" vowel, which is then followed by a "k" consonant. The "t" sound is then pronounced, followed by the "ɛm" sound, which is a combination of the "ɛ" vowel and a nasal "m" sound. The remaining sounds include "bri" and "aɪ", which make up the word's last syllables. Overall, it's an unusual but phonetically sound spelling.
In bot., those leguminous plants which have the embryo straight in the axis of the seed.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.