Grape Leaf Pearl Glands

I read about these things called "pearl glands" in Ronald Jackson's book on Wine Science on page 60:
The outer tissues of the young stem consist of a layer of epidermis and several layers of cortical cells. The epidermis is initially photosynthetic and bears stomata, hair cells and pearl glands, especially along the veins. Most cortical cells also contain chloroplasts and are photosynthetic.
Yesterday, when I was up close and personal with one of our grapevines, I saw what Ronald Jackson was talking about:

The hairs as well as the exudate from the pearl glands were clearly visible. This morning, I was talking with my husband about these pearls that I saw and he mentioned that he also saw these spherical pearls on the bottom of the leaves that he was looking at. Yes, young leaves also possess leaf hairs and pearl glands.
References:
1. Ronald Jackson, Wine Science Third Edition, Elsevier, 2008, pg. 60 and 62.