By the looks of them, many algae and plants may appear the same, at least superficially. However, they can be structurally and functionally miles apart. Due to their differences, algae and plants are classified into two different kingdoms known as "protista" and "plant," respectively. Thus, it is useful to know the main differences between algae and plants which lead them to be classified under two different kingdoms and to be considered as having different origins than being "monophyletic," or originating from a common ancestor.
Microscopically, all plants are considered as having more than one cell or else as "multi-cellular." However, in contrast, algae can be unicellular, multi-cellular or even colonial. Macroscopically, plants consist of roots, stems, leaves, flowers, fruits, seeds, cones etc., while certain plant-like algae would consist of structures known as holdfasts, stapes and blades, which are structurally and functionally different from the components present in the plants. Furthermore, the tubing system within a plant will allow the flow of nutrients and water from its root system through to all other structures in the plant. In contrast, each algae cell is required to obtain or produce its own water and nutrients rather than rely on any other structure or a vascular system such as that present in plants.
Plants are generally immobile in that they cannot move from one place to another because of the root system. Although some algae consist of root-like structures known as holdfasts, they are able to move either with the water current or else by using structures known as "flagella." Some algae can also move by pushing against the surface using their own bodies.
Plants are generally found on land, although there are few plants that can even remain submerged under water. Algae, on the other hand, are found typically in wet conditions or more commonly in water. However, they can be present even in the harshest of conditions, such as on rock surfaces, in very cold climates with icy conditions or even on the surface of certain marine animals or animals living under wet conditions.
The reproduction process in plants can be rather complex and would require flowering, seeding, spread of germinated seeds etc. Thus, plant reproduction may also require the assistance of wind, rain or even the birds. However, algae reproduction is a relatively simple process involving tiny spores, or else small pieces of the algae itself, by a process of replication.
Thus, even though certain algae and plants may be comparable in structure, they can differ from each other in many different ways. These differences may be related to their structure, mobility, reproduction, location as well as in relation to certain other functions. Thus, classifying the two in two different kingdoms is justifiable.