This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
We were nearly remarking that the Sow-thistle is one of the most beautiful of our native flowers, but remembering that we have already applied that observation to several species, we will alter the formula and say it is among the most handsome. Certainly no one who sees it growing is likely to pass it by without plucking some of the flowers, though they will be disappointed in these flagging and losing their beauty before home is reached. We have three native species, of which this is undoubtedly the finest, the stem growing to a height of three or four (or, as we have found it in Surrey, over five) feet. It is a perennial, with a large creeping rootstock, which sends off runners. The stem is hollow, milky, and clasped by the bases of the finely cut leaves. These are deeply lobed, and edged with sharp teeth; the lower leaves have stalks, the upper have not. The unopened involucre - for this again is a Composite - will strike the finder as being singularly square; it is covered all over - as are the stems also - with short hairs with glandular tops of a golden yellow. The expanded flower-head is about two inches across, and is composed entirely of ray-florets. The plant will be found flowering in or around cultivated fields in August and September. The other British species are:
I. Marsh Sow-thistle (S. palustris), now all but extinct, and found only rarely in the Eastern counties of England and Kent. It is taller-growing than arvensis, the stem sometimes reaching nine feet, but the flowers are only half the size of that species.
II, Common Sow-thistle (S. oleraceus). A common annual in every field and waste. General character of plant very similar to arvensis, but smaller. Stem, two to three feet in height, without (or rarely with) the glandular hairs. Flower-heads many, not exceeding an inch in diameter. June to September.
Name supposed to be derived from the Greek, sonthos, hollow, in reference to the fistular stems.