180 THE EVOLUTION OF PLANTS 



At one time the secondary growth of the 

 Lepidodendrese, and especially of Sigillaria, of 

 which it was supposed to be characteristic, misled 

 many botanists into thinking that these plants 

 were Gymnosperms and not Cryptogams. We 

 now know, thanks chiefly to the work of William- 

 son, that the formation of secondary tissue is 

 by no means peculiar to Seed-plants, but ex- 

 tended, in Palaeozoic days, to every group of the 

 higher Cryptogams. In our own time we only 

 find reminiscences of the old capacity for un- 

 limited growth, in Isoetes and a few other cases. 

 No doubt secondary growth was to a great ex- 

 tent correlated with the tree-habit, though it was 

 not limited to trees then, any more than it is 

 among Dicotyledons at the present day. Sec- 

 ondary growth by means of a kind of cambium 

 goes on in some of the larger seaweeds (Lami- 

 naria, etc.) at the present day, and was even more 

 extensive in a huge Palaeozoic Alga, named 

 Nematophycus. 



Each Lepidodendron leaf, as a rule, received a 

 single vascular strand from the stem; the leaf- 

 strands were comparatively small, and passed out 

 without causing any gap in the ring of wood from 

 which they started. This is another impor- 

 tant difference between Lycopods and Ferns; in 

 the latter there is almost always a leaf-gap, where 

 the strand of a leaf passes out, because of the 



