130 The Botanical Gazette. [April, 
The roots, protected somewhat from the extreme condi- 
tions prevailing above ground, have not been found to show 
notable peculiarities. They often run very deep, are often 
tuberous for water-storage, are rarely aerial. But in the 
shoot the necessity for condensation, i. e., for surface reduc- 
tion in proportion to bulk, has operated to lessen, even to the 
point of suppression, the branching and leaf-formation, has 
brought about very special form-conditions, probably unique 
relations of stem, leaf and axillary bud, and a very finely- 
adapted series of water-holding tissues. ? 
As to the tissues, it is enough here to say that the charac- 
teristic xerophilous appearances are a strong cuticle, thick 
epidermis, perfect cork, sunken stomata; collenchymatous 
hypoderma; deep palisade layers; great development of pith 
and cortex which consist of large round splendidly pitted 
water-storing cells, often containing mucilage; a fibro-vascu- 
lar system in general simple in its make-up, lacking annual 
rings, composed as to its xylem part of strongly ringed and 
spiralled tracheids which are often collected into gland-like 
masses, the whole system conforming closely to the exter- 
nal form and following its morphological changes. 
In external form, there is every variation from the leafy 
shrubby Peireskia to the ribbed columns of Cereus, the flat 
joints of the Platopuntie, the phyllocladia of Epiphyllum, or 
the tubercled spheres of Mamillaria, and everywhere are clus- 
ters of spines, in definite relation to which arise the flowers 
and new branches. How is this medley of structures to be 
brought into homology with the ordinary stem and leaf con- 
dition of other flowering plants? Happily these questions 
have been mostly solved. All Cactacez have leaves which 
show instead of the ordinary division into blade, petiole, etc-, 
a division into blade and swollen base flattened to the stem.’ 
The blade may persist for a season as in Peireskia, fall away 
early as is usual in Opuntia, remain very small as in Cereus, 
or microscopic as in Mamillaria. The axillary bud develops, 
not strictly in the axil, but upon the leaf-base, having been 
of ada 
For 
three phases of one, the first asking what a structure is, the second by what 
steps it has come to be what it is, and the third why it is ‘what it is. 
* A xerophilous characteristic, found also in Euphorbiacez and others. 
