A stromule is a microscopic structure found in plant cells. Stromules (stroma-filled tubules) are highly dynamic structures extending from the surface of all plastid types, including proplastids, chloroplasts, etioplasts, leucoplasts, amyloplasts, and chromoplasts."

Source : Wikipedia

They are ubiquitous, being found extending from the surface of plastids in all multicellular plants - however :

"[…] their function is still largely a matter of conjecture. They may increase the plastid surface area to facilitate movement of materials into or out of plastids, be involved in sensing the cellular environment, and/or have signaling functions due to close apposition of stromules and nuclei, plasma membrane and other cell organelles. Stromule formation appears unrelated to chloroplast division or to light-intensity-dependent chloroplast movement."

Source : Plastid Development in Leaves during Growth and Senescence pp 169-186

Currently, there are at least ten hypotheses regarding their possible functions, and several more suggesting how they might form. See : Plant Physiology,Volume 176, Issue 1, Pages 128–137

Also see : Endophytesplugin-autotooltip__plain plugin-autotooltip_bigEndophytes

Endophytes (literally “inside plants”) were first found in 1809 by examining plant sections under the microscope. They are fungal, or bacterial, or viral inclusions in plant tissues – 'foreign' materials, found in very large numbers. They're permanent fixtures, being transmitted from generation to generation of plants either through the seeds, propagation, or by ‘infection’.