Exegesis:
gar ‘for,’ introduces the principle which underlies the statements of v. 43.
ek tou idiou karpou ginōsketai ‘is known by its own fruit.’ The present tense is used in order to express a general truth. ek indicates the source of knowledge.
ou gar ex akanthōn sullegousin suka ‘for one does not gather figs from thorns.’ gar introduces self-evident facts as examples of the principle which v. 44a expresses. sullegousin is used in an impersonal sense. sukon.
akantha ‘thorn plant.’
sullegō ‘to collect,’ ‘to gather,’ a very general word.
oude ek batou staphulēn trugōsin ‘nor does one pick grapes from a thorn bush.’
batos (also 20.37) ‘thorn bush,’ ‘bramble bush.’
staphulē ‘bunch of grapes,’ ‘grapes,’ usually referring to ripe grapes.
trugaō ‘to gather (ripe fruit).’ As compared with sullegō it is a much more specific term.
Translation:
Each tree is known by its own fruit may have to be expanded, e.g. ‘one knows trees by looking at the fruit that each (of them) bears,’ ‘one knows a tree when one knows its fruit, or, the fruit it produces.’
The second for is best rendered, ‘of course,’ ‘as everyone knows.’
Figs. The reference is to an edible and valued kind of fruit. A more or less literal rendering is often possible but in some languages another fruit has to be substituted, either in all occurrences or specifically in this verse, e.g. in Tae’ 1933 (‘bananas’), Huixtec Tzotzil (‘apples’).
Thorns. The name of any kind of thorny wild growth, shrub or tree with no fruits, or inedible fruits will do.
Grapes. Where ‘wine’ is known but ‘grapes’ are not one may say ‘wine-fruits’ (Bahasa Indonesia); elsewhere the name of edible berries, or of the fruits producing the beverage used instead of ‘wine’ (for which see on 1.15) may be acceptable, or again a local equivalent not connected with wine, e.g. ‘peaches’ (Huixtec, Tzotzil), ‘guava (Psidium Guajava)’ (Tae’ 1933).
Picked, more specific than, but virtually synonymous with, “gathered”. Where no synonymous verb is available it may be omitted in the second clause, as done in Tae.’
Bramble bush, synonymous with “thorns”, ‘thorn-bush,’ symbol of unfruitfulness.
Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.