How does a lack of lignin limit the height of bryophytes?
Bryophytes lack the conventional vascular tissues which usually contain the substance lignin. Without vascular tissues, these plants have no means of transporting water and/or nutrients from their lower organs to the higher organs which explains their limited height.
The vascular tissues (xylem and phloem) of most higher plants are made up of schlerencyma cells which are basically dead plant cells with thick lignified cell walls whose main function is the transport of water or substrates. Bryophytes lack these structure and therefore, if they have the height of a conventional plant, they would have no means of supplying water and nutrients across their systems. The minute height and size of these plants help them survive because their organs are very close to the water source and thus a transport system is unnecessary (since they are very small, they do not need much support and so the need for lignin is also minimized).