"And Jacob took for himself rods of fresh poplar, and of almond and plane tree" (Genesis 30:37-39): a fresh white branch, of almond, and of plane. Thus Jacob our father would place the rods inside the water troughs. The animal would come to drink and would see the rods and recoil backward, and the male would mount her, and she would give birth to one like it. Rabbi Hoshaya said: the waters became seed within their wombs, and they lacked nothing but the form of the offspring. Rabbi Huna of Bet Horon said: the ministering angels would carry off from Laban's flock and place among Jacob's flock. This is what is written, "Lift up your eyes now and see all the rams" — it is not written "that climb," but rather "that are made to climb" of their own accord [the unusual form hinting they were brought up by another hand]. Rabbi Tanhuma said: by a rush of rains. Our rabbis say: by clouds of glory.
The Peeled Rods and the Angels Who Bred Jacob's Flocks
Yalkut Shimoni on Torah 130:9
(בראשית ל לז-לט) וַיִּקַּח לוֹ יַעֲקֹב מַקַּל לִבְנֶה לַח וְלוּז וְעַרְמוֹן. חֹטֶר רָטִיב חִוָּר, דְּלוּז, וְרַדְלוּב כָּךְ הָיָה אָבִינוּ יַעֲקֹב נוֹתֵן אֶת הַמַּקְלוֹת בְּתוֹךְ הַבּוֹרוֹת שֶׁל מַיִם. הָיְתָה הַבְּהֵמָה בָּאָה לִשְׁתּוֹת וְהָיְתָה רוֹאָה אֶת הַמַּקְלוֹת וְהִיא נִרְתַּעַת לַאֲחוֹרֶיהָ, וּבָא הַזָּכָר וְרוֹבְעָהּ וְהִיא יוֹלֶדֶת כַּיוֹצֵא בָּהּ. אָמַר רַבִּי הוֹשַׁעְיָא, נַעֲשׂוּ הַמַּיִם זֶרַע בְּתוֹךְ מְעֵיהֶם וְלֹא הָיוּ חֲסֵרוֹת אֶלָּא צוּרַת הַוָּלָד. אָמַר רַבִּי הוּנָא דְּבֵית חוֹרִין, מַלְאֲכֵי הַשָּׁרֵת הָיוּ טוֹעֲנִין מִתּוֹךְ צֹאנוֹ שֶׁל לָבָן וְנוֹתְנִין בְּתוֹךְ צֹאנוֹ שֶׁל יַעֲקֹב הֲדָא הוּא דִּכְתִיב "שָׂא נָא עֵינֶיךָ וּרְאֵה כָּל הָעַתּוּדִים" עוֹלִים אֵין כְּתִיב כָּאן, אֶלָּא "הָעוֹלִים" מֵאֲלֵיהֶם רַבִּי תַּנְחוּמָא אָמַר, שֶׁטֶף שֶׁל גְּשָׁמִים רַבָּנָן אַמְרִין, עַנְנֵי כָּבוֹד.