As with all variables of life intelligence is an adaptation, suited for the lifestyle of the specific organism. For instance, research on birds demonstrated that populations (of the same species) which lived through harsher winters showed enhanced problem-solving ability. That withstanding, there are generalities which apply to all.
The mind is a symbolic pattern matcher. All cognition and learning are a play of concepts. A concept is a locus of cohesive conception in a context of related concepts. Hence, a concept is a mental tensor: all information is relational. Conceptual contexts are organized hierarchically, from general to specific, with associative links.
Microbes live the purest mental lives. Their world is of molecular interactions. They must distinguish between foodstuffs, toxins, and friend from foe. Microbial communications are molecular, and often include messages made by fabricating specific molecules.
Microbes also understand genetics at the molecular level. This aspect of microbial intellect is utterly alien to animals, who have no such ability.
Plants extend microbial cognitive acumen into the richest possible mental life. Plant molecular savvy extends into creating concoctions designed to enchant potential friends or foil foes in the most devious ways – beyond our ken in how such feats may be accomplished, let alone in how plants know so much about the intimacies of other life forms.
Along with the social categories of friends, competitors, and community, plants also have suppliers, symbionts, and pollinators to constantly consider and manage. The advent of angiosperms greatly enhanced the social possibilities and demands of daily life. Floral relations are considerably more complicated than those fauna face, and involve more subtle communications and interactions.
Facets of intelligence are life-history variables. The solutions to many challenges faced in life may be instinctual. This is true for every organism, which also has as precocious knowledge how to learn in specific realms and in certain ways. Learning is needed to adapt to situations that are habitat-specific, most notably food sources and their procurement; even as the basic algorithms for foraging may be innately encoded (e.g., Lévy walk).