30 
TRANSACTIONS OF TIIE TEXAS ACADEMY OF SCIENCE. 
is soon followed by another, and then others. Fig. 1, iii. Growth con¬ 
tinues for quite a while this way, and if the plant is a Cactus, these 
tubercles wdth their spines assume the adult characters, and the process 
continues throughout life. But not so with the other genera. In Echino- 
cactus, after a while the tuberclea seem to be flattened considerably at 
the base. This is due to the fact that they increase in size 
in a nearly perpendicular direction, but not in a vertical. 
This increase continues until all the space between two 
adjacent tubercles is filled with substance of the plant 
body, and ribs are formed. The spines then assume spe¬ 
cific characters, and the changes cease. A diagramatic 
illustration of the idea is shown in Fig. 2, giving an out¬ 
line appearance of a single rib. 
These ribs may become vertical or spiral in thefollowing way: The 
arrangement of the tuber¬ 
cles is in the form of a 
quincunx, and if the 
growth is such that the 
tubercles unite as indi¬ 
cated by Fig. 3, vertical 
ribs are formed; but if the 
union occurs as in Fig. 4, 
the ribs are spiral. 
In the case of Cereus and Opuntia the young tuberculate form begins 
to elongate, and the appearance is such that both would be taken for 
some form of cereus. They have cylindrical bodies and soft spines, and 
are more or less ribbed. Spines of cereus now assume their adult form, 
and opuntia begins to differentiate into its varied forms. 
These observations lead me to the following conclusions: The type of 
the caetaceae is a globose or oval plant body bearing tubercles on its sur¬ 
face, and is represented by the various forms of the genus Cactus, which 
has been lately re-established by Prof. Coulter instead of the Mammil- 
laria of Hawthorn. This type has given rise to two forms, one a ribbed, 
globose or flattened body, as represented in Echinocactus; the other an 
elongated ribbed or slightly tuberculate body, which in turn differenti¬ 
ates into the various forms indicated by Cereus and Opuntia. It will be 
noticed that mature plants of these genera often exhibit a tendency to 
tuberculate forms, especially if it forms branches. With this fact before 
us, it will not be altogether unsafe to say that probably Phylocactus, 
which in the young shoot, or branch, shows a cylindrical, slightly tuber¬ 
culate form bearing closely oppressed leaves, is nearly related to Opuntia. 
Observations have not been made on the seedlings of this genus. 
