1895.] Present Problems in the Cactacee. 133 
cease to form spines and before they begin to form leaves. ® 
These transitions have been found in Opuntia and Echinopsis. 
Do they occur in other genera? Do they occur on the bases 
of flower-branches? Do any monstrosities, between leaf and 
spine, occur? 
In the genus Opuntia and confined to it, in addition to the 
spines, occur the fine barbed bristles, produced on the inner 
side of the axillary vegetative point. Transitions between 
these and the spines are to be found on old sprouts of some 
Opuntias and the two are doubtless homologous. Are transi- 
tional forms elsewhere to be found? 
The spines of many forms are white in color and weak in 
texture, even to becoming hair-like as in Pelocereus senilis 
and many others. The epidermal cells of such spines are 
usually provided with openings through which ame may be 
seenundert absorbed, the air causing 
the whiteness being expelled, but no trace of a tissue for conduct- 
ing such water to the living parts has been found. Why do 
the epidermal cells absorb water? Is it conducted into the 
stem? The spines of Echinocactus species show a marked 
cross-banding, due to alternation of clearer and opaquer nett 
the former being of larger diameter than the latter. Them 
croscope shows that the cell-cavities in the darker bands con- 
tain air, while the clearer lack it. |. This seems to be an inci- 
dental growth condition. Upon what does it depend? What 
relation do the bands bear to the age of the spine? In the 
Cylindropuntiz, each spine is commonly covered by a thin 
easily separable sheath. This has been found to consist mor- 
Phologically of a layer of hairs grown together.® It cannot 
be necessary to protect the spine in its development, for 
others grow equally well without it. Is it of use or was it 
formerly of use? Is it a growth phenomenon?’ In some Mam- 
illarias the epidermis of the spines extends out into hairs so 
that they become feather-like, and this is the common form 
for them in the seedlings throughout the family. | Why are 
they of this form? Do they help to prevent evaporation? 
Very little is known of the biology of the spines. It is as- 
Sumed that the strong ones protect the plants against animals 
T y be found only when the branches are very young, indeed just as 
— pe prec signs of appearance, for the transitional leaf-spines wither and 
On very early. 
2 ®*The discussion of this —— with a other topics here mentioned may be 
ound in my above cited paper in ‘‘Flor 
