Here in the earliest extant text, the verb form partook reads as the past participle for partake, probably not as the simple past-tense form (the text is equivalent to “if Adam had ... partook of the tree of life”). The original text permitted conjunctive constructions like this one (see the discussion under 1 Nephi 5:8). For discussion of the past participial form partook, see under 1 Nephi 8:25. Here in Alma 42:5, the 1920 LDS edition replaced the nonstandard partook with the standard partaken. The critical text will restore the original partook.
Summary: Restore in Alma 42:5 the original partook, here the past participial form of the verb partake.