396 Dar bishire, — Observations on Mamillaria elongata . 
been recorded from Limapan, and las Ajuntas on the river Moctezuma, near 
Ixmiquilpan, Meztitlan, between Zucualtepan and the river Toliman and 
the Rio Grande. It is supposed to occur also in Chihuahua, but from this 
latter locality Schumann has not seen any authentic specimens. Hidalgo 
is a state or province of Mexico right in the centre of the high plateau on 
the top of the Mexican mountain range. 
Schimper gives the following as the conditions obtaining in the 
Mexican plateau ( 24 , p. 675). The climate generally is dry, though 
moister than that of the North American deserts. The annual rainfall 
appears never to be less than 50 cm., which is considerably above that 
of the typical desert climate. Even with a high temperature prevailing 
one would not expect to find a very poor vegetation except on soil very 
permeable to water. 
Karsten gives us a very useful glimpse of the conditions prevailing in 
these localities ( 24 , p. 678). In the summer the days are warm and 
sunny, little rain falls, and the nights are relatively very cold. In winter 
snow falls, but it very soon melts away. 
It is difficult indeed from the data which are given in the literature of 
the subject to really get an accurate idea of the Mexican climate. It seems 
to be very hot during the day and cold during the night, in the summer. 
The rainfall, though low, is not a desert rainfall. But, as Schimper remarks, 
the height of the Mexican deserts makes them subject to many of the 
desiccating influences of an alpine climate. Edaphic influences, as yet not 
at all properly understood or even known, also probably are at work in 
determining the nature of the plant-forms. Volkens’s observations on the 
Arabian-Egyptian desert are very important, great stress being laid by him 
on the strength and clearness of the light ( 33 , p. 15). 
Walther briefly, but I think very well, summarizes the five conditions 
which in the desert are chiefly responsible for the poor development of the 
vegetation. They are the following ( 35 , p. 79) : (1) The scarcity of rain 
and dew ; (pi) the strength of the sun’s rays ; (3) the violence of the dry 
winds ; (4) the looseness of the particles of soil ; (5) the salinity of the soil. 
Armed with these very few data I wish now to offer an explanation 
as far as possible of the structures met with in Mamillaria elongata. Of 
these some are not directly connected with the actual climatic conditions, 
but to make this paper more complete they will also be referred to. 
The external and internal structure which a plant exhibits is mainly 
due to the way in which it has responded to the influence of external 
conditions. Ontogenetically or phylogenetically the form which a plant 
represents is an expression of those external conditions which in some way 
influence adversely or the reverse those functions which are carried out in 
the plant and which are of vital importance. Some structures met with 
in a plant may no longer be of use to the plant ; they may in fact be merely 
