Which modified stem lacks vascular tissue?

The correct answer is prickle, which is a type of modified stem that lacks vascular tissue. Prickles are superficial outgrowths from the epidermal tissue of a plant and do not form part of the woody structure found in stems. They are typically found on the surface of stems or leaves and serve various protective functions.

In contrast to prickles, tendrils, thorns, and bracts all involve some degree of vascular tissue. Tendrils, often modified stems or leaves, aid in climbing and support but contain vascular tissue to support their growth and function. Thorns are modified branches or stems that usually have vascular tissue, providing structural support and protection. Bracts, which are modified leaves often associated with flowers, can also contain vascular tissue to support their structure and function.

Thus, prickles are unique among the options as they are extension of the epidermis and do not involve the vascular tissue that is present in the other modified stem types.