Martin (1992: 103, 104):
Generic groups in other words do not depend on their context in the way that specific groups do; unless their realisation involves demonstratives or pronouns (e.g. they and these desert water-tanks in [3:17] below), their context is in effect simply that of knowledge of the language being used.
[3:17]The cacti have extensive root systems spreading in all directions — sideways and downwards — to soak up as much water as possible when it rains. They are able to swell to store water, and they then use this water over long periods of drought. A thick waterproof covering protects these desert water-tanks with their soft pulpy cells, and their leaves are often reduced to thorns to cut down on water-loss and protect the plant from animals that might otherwise eat it for its moisture.
 As explained in previous posts, the term 'generic groups' confuses both the referent ('generic') and the nominal group featuring the reference item ('group') with the reference item itself.
 As explained in previous posts, 'depend on' misconstrues the textual relation between reference item and referent as a logical relation of dependence. The inconsistency is one of metafunction.
 To be clear, in text [3:17], the personal reference item they and the demonstrative reference item these both mark anaphoric co-reference to the cacti. To be clear, the cacti construes the members ("manifestations") of the class 'cactus', not the "generic" class itself.
 To be clear, nominal groups that do not include a co-reference item, personal or demonstrative, do not include a reference to an identity that needs to be recovered from the co-text.