Dar bishire. — Observations on Mamillaria elongata. 397 
of morphological interest, indicating the persistent remains of an obsolete 
organ or member. The more adverse the conditions are, however, the more 
likely are we to find in a characteristic plant-form peculiar physiological 
structures which owe their presence to the adverse conditions directly, the 
less likely are we to find any useless members. 
The conditions of the Mexican desert are very unfavourable, and we 
get there a very typical and characteristic plant-form, represented by almost 
the whole of the Cereoideae group of the Cactaceae. I think it very 
probable that the whole structure of these plants reflects almost in its 
entirety the influence of the prevailing adverse external conditions of the 
desert. 
Of the vital processes which are being carried out in the plant, two, 
I think, may be considered as depending most on external conditions. The 
structure of the plant, as representing any particular plant-form, therefore, 
will be the more modified from what we can call the normal form, namely, 
a green land-plant with well-expanded foliage leaves, the more adversely 
external conditions affect the carrying out of these functions. 
The first of these two functions includes all those processes which go 
to make up, or which take part in what is generally known as, the 
transpiration-stream : namely, the absorption of water with the raw material 
from the soil in solution, the carrying of the latter to the green leaves, their 
deposition in the green cells of the leaves, followed by the giving off of the 
greater part of the water thus brought up from the cell-surfaces in 
the intercellular spaces of the mesophyll. The second includes all the 
processes necessary for the carrying out of photosynthesis. 
(b) Physiology of Mamillaria elongata. 
Mamillaria elongata grows in dry and hot places and one might expect 
to find a fairly large root. Judging, however, from the plants I have been 
able to examine, the root is rather short and fat and but little branched 
(PI. XXV, P"ig. 3). It is impossible, however, for me to say what the root 
would be like if a plant were allowed to grow under natural conditions. 
The root shows one very striking feature in the structure of the xylem. 
It consists of annular tracheids throughout, disregarding for the present the 
parenchymatous cells surrounding the wood tracheids. The spiral nature 
of the thickening in the protoxylem-elements of young plant-members of 
both root and stem has been thought to be of use in allowing the tracheids 
or vessels to elongate during the growth of the plant, without rupturing the 
whole tracheidal thickening ( 31 , p. 469). 
If this be the case, the annular tracheids of Mamillaria might be 
considered an adaptation to a possible and very likely shrinkage of the 
whole plant and especially the root during the dry season, and a subsequent 
swelling up again and elongation during the moist season. A shrinkage of 
