Lab Girl is a story of survival—the survival of plants and the survival of humans in a harsh and challenging world. In Jahren’s view, plants are creatures much like humans, with wills of their own, who struggle to survive in the face of adversity and turmoil just like people do. Throughout the book, Jahren weaves the lives of plants with the lives of people, and she personifies the plants to convey their life force and compare it with the life force of humans.

Most significantly, she links the struggles of plants with the struggles she herself faces as she fights mental illness and prejudice in her attempt to establish herself as a scientist. Several prominent themes of the book are survival and the continuing fight for survival despite discouraging odds.

As a child, science and play are inseparable for Jahren, so it follows that, as she grows, she draws more comparisons between the scientific and personal parts of her life. From the time of her earliest memories, the two categories are always intricately connected. Over the course of her career, Jahren gains a fuller understanding of a plant’s struggles and a fuller understanding of her own. Jahren’s understanding of plants in this way helps her identify with them and bond with them. Similarly, the natural bond she feels with plants leads her to gain a better understanding of how they grow and thrive.

The structure of the book supports Jahren's thematic interweaving of plant and human life. The book is split into three sections, each named for parts of plants that are emblematic of different stages in their life cycles: "Roots and Leaves," "Wood and Knots," and "Flowers and Fruit." Within each section, Jahren alternates the narrative of her life, often written in longer chapters, with short, two- or three-page meditations on aspects of plant life and cycles (such as, for example, seeds, leaves, deciduous trees, "resurrection plants," and so on). These sections are often clearly juxtaposed with stories from her own life in order to establish likeness. For instance, in the third part of Lab Girl, Jahren follows a meditation on plant reproduction and the importance of touch and contact with a chapter that recounts how she meets her husband, Clint. Soon after, she includes a vignette about plant "growth curves" before describing her own difficult experience with pregnancy.

By interspersing her own story with poetic essays on plants, Jahren draws comparisons between herself and these other forms and emphasizes the underlying likeness among life of all kinds: however different we may seem at first glance, we all are born, live, grow, and die.