84 JOUE^AL OF THE EOYAL HORTICULTUKAL SOCIETY. 
The AiTow-liead (Sagittaria) follows suit, for as long as the leaf grows 
in deep water it has only a long, flat phyllode. This bears a blade 
only when it approaches the surface, which is at first oval, then when 
clear from the water it is hasiate in shape. 
So, too, some Monocotyledons have two cotyledons, as Asparagus 
and Buscus, one being rudimentary. 
Some ]\Ionocotyledons, as our Lords and Ladies (Arum maculatum), 
have a blade with reticulated venation, a remembrance of its ancestral 
Dicotyledon. 
Hence the so-called "blade" of a linear-leaved Monocotyledon, as 
of a grass-blade, is^strictly homologous with the stalk, and is essentially 
a phyllode which does duty for the blade. 
The reason why the parts of the flowers of Dicotyledons are in fives, 
and those of Monocotyledons are in threes, is explainable by the fact 
that when leaves are in pairs and decussate, on becoming alternate 
the arrangement results in " cycles " or groups of fives. But starting 
from a single cotyledon, the arrangement follows, which contains three 
leaves in a " cycle " or whorl. The two systems are therefore con- 
sequences of there being two and one cotyledon respectively. 
The general conclusion is, that to look for the prime cause of 
evolution in plants we must turn to the protoplasm, and especially the 
nuclear chromosomes, as the active agents ; not only in transmitting 
hereditary qualities, but in having the power to be affected by ex- 
ternal forces ; responding to which, they set to work to construct cells 
and tissues better suited to new environments ; and if the action be 
continued for some generations, the structures become relatively fixed 
and hereditary, until some new conditions bring about fresh variations. 
