400 Dar bishir e. — Observations on Mamillaria elongata. 
The narrow tracheids in the root and stem, and those leading into the 
tubercles, are primarily conducting elements ; all the other larger ones, 
besides conducting, will also store. This view held by Strasburger seems 
to fit in well here ( 31 , p. 469). The tracheids in the tubercle are also 
giving off water. Water is being taken from them by the osmotic action 
of the surrounding cells. The cells surrounding the large tracheids of the 
stem, however, probably have no such function. Their cavities, as already 
pointed out, are small (PI. XXVI, Fig. 21). 
From the figures given above, the goal of the upward current of water 
is always a large tracheidal cell. Strasburger mentions that the current 
of water is always towards the smaller cavity ( 31 , p. 873), an observation 
which does not accord with my observations on Mamillaria elongata 
recorded here. 
Mamillaria elongata growing in a dry desert region would naturally 
show a number of xerophil structures. Before however referring to these, 
it will be necessary to describe the arrangements for carrying out photo- 
synthesis. 
We have a very well-developed palisade-tissue on all sides of the 
tubercle (PI. XXV, Fig. 15). The cells of this are supplied with raw 
material from the soil directly by the cortical bundles, and, more indirectly, 
by the medullary bundles. It may at first sight appear remarkable that 
the rows of palisade-cells should run at such a definite oblique angle with 
the epidermal layer of cells. The strongest light that falls on the palisade- 
tissue very probably impinges on the plant at this same angle. Light 
coming vertically down from the sun would not reach the tubercles low 
down on the plant (PI. XXV, Fig. 3), but would be caught by those higher 
up, which are nearer the growing-point. But these tubercles are placed at 
a slightly different angle, with regard to the axis of the plant, than the 
lower ones. Their palisade-tissue also has its cell-rows at a different and 
again probably correct angle, in order again to catch the rays end on (PL 
XXV, Fig. 18). The same explanation serves to make clear the meaning 
of the rows of palisade-cells on the underside of the tubercles. They catch 
the also very strong light which is reflected from the surface of the soil. 
Whether this is glistening sand or not I cannot say, but it very likely 
frequently is in the natural habitat of our plant. This certainly appears to 
be the case with Cerens peruvianus and other Cactaceae as depicted by 
Karsten and Schenck ( 16 , Pis. XXXIX to XLVIII). 
The plastids are generally found on the radial walls of the palisade- 
cells, except on occasions when they congregate around the nucleus, an 
occurrence noted and figured already by Schleiden ( 25 , p. 6 , PI. VII, Fig. 3). 
To the inside of the cortical bundles the large parenchymatous cells are 
arranged more like the ordinary cortical tissue of the main body of the 
plant (PI. XXV, Fig. 13). 
