why algae are not plant?

Algae is plant- like organisms
but algae
no vascular tissues
no roots or false roots
no lignin
no mesophyll cells, paraphyll cells, no cortex, no bark no epithellial cells
(plants have)

floats but not attached to substratum( plants attach to substratum)

micro-size( plants are much bigger)

Algae ae a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. The US Algal Collection is represented by almost 300,000 accessioned and inventoried herbarium specimens.[3] The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in land plants