ORIGINATION OF SELF-GENERATING MATTER 617 
reproduction could not exist on land areas independently. It is also 
to be noted that many forms peculiar to swampy areas at the present 
time display reduced shoots and leaves of a specialized structure due 
to the action of certain constituents in the substratum, that they are 
known as “swamp xerophytes” and if brought to light as fossils might 
give the impression of having lived in an arid climate. 
Fic. 5.—Types of plants from the Tucson desert, where two distinct yearly periods 
of maximum precipitation occur. In addition to the morphological reduction of the 
shoots and leaves, the capacity for the accumulation and retention of water has reached 
an enormous development. A group of sahuaros (Carnegiea gigantea) on the left; a 
single bisnaga (Echinocactus Wislizenii) on the right. ‘The last-named plant has a 
supply sufficient for a dozen years’ activity in its tissues. 
The leaves of conifers very probably represent a specialization 
adapted to existence in a dry atmosphere, yet it is notable that the 
greater majority of surviving species live in soils in which the occur- 
rence of moisture is not that of the desert. 
The swollen stems of the Bennettitales offer the strongest sugges- 
tion of desert forms, and their structure and reproduction would 
