These technical drawings of a lettuce root system are from Root Development of Vegetable Crops*, first published in 1927 and now in the public domain. This is an incredible book that I just discovered. The text is like a complete gardening course delivered from underground. The drawings record direct observation, the result of years of root excavation. Over 30 (North American) common garden veggies are covered, a chapter each. I can hardly describe how satisfying and…enlightening it is to simply look at page after page of painstakingly drawn root systems! Here, the top two pictures are lettuce at two months and then at three months and flowering. Each square in the grid is one foot. The little side-by-side illustratation shows 3-week-old seedlings, grown on the left in loose soil (nearly 2′ down!!), on the right in compacted soil (and to think, I have 3-week seedlings in tiny plug sheet cells, 2-1/2″ deep!). One look at this and your mind expands!
*I downloaded it from the fantastic Soil & Health Holistic Agriculture Library, an online repository for dozens of excellent books, mostly from 1910-1960, and all entirely free—there are many organic farming classics, tons of great, practical stuff!