Dar bishire . — Observations on Mamillaria elongata. 407 
sunshades, some other structure in the plant takes on the paraheliode 
function. 
I have been able to examine only a few species from this last point of 
view. The results obtained are not satisfactory, as long as it is impossible 
to take into very careful consideration the details of the natural conditions 
which surround the plants in their native habitat. 
I am therefore simply quoting a few of the plants examined, in order 
to show on what lines such an inquiry could, I think, be carried out with 
advantage. 
Paraheliode 
effect of 
spines. 
Depth at 
which 
chlorophyll 
begins. . 
Total wall- 
thickness 
in this 
depth. 
Cuticle. 
Leuchtenbergia principis 
Very small 
308-5 n 
75 m 
3-5 M 
Echinocactns cylindraceus 
Very strong 
185-5 fi 
127 IX 
i7-5 M 
Anhalonium Williamsonii 
Absent 
141-3 
— 
7M 
Echinopsis Muller i 
Strong 
133-5 M 
S^M- 
3 ix 
Echinocactns cornigerus 
Weak 
105 M 
7°M 
3'5 M 
„ gibbosus 
Weak 
IOI n 
16 ix 
2-5 M 
Cereus Baumanni 
Weak 
7 6-5 m 
10-5 ix 
2 M 
Mamillaria B ocas ana 
Very strong 
24\5M 
2-5 M 
1-5 M 
„ pusilla 
Very strong 
31-5 f* 
3-5 M 
1-5 M 
„ elongata 
Very strong 
i9M 
7-°M 
1-5 M 
From this table, which refers only to very few species, we can see the 
degree to which the depth varies at which the chlorophyll begins. The 
cuticle is generally found to be thin. I have not been able to detect 
the relation which may exist between the spines and the layers overlying 
the chlorophyll. It is, I think, more likely that the paraheliode effect 
of the spines should be added to that due to the white protective layers of 
epidermis and hypoderma, and these two features together should be brought 
into relation with the surrounding conditions. 
Michaelis refers to the thick cuticle of Echinocactns and the thin cuticle 
of Mamillaria , but he mentions the hypoderma of the latter as consisting 
of tall, slightly thickened cells (19, p. 26). In the few plants of this genus 
which I have examined they appear flat (PI. XXV, Fig. 15). 
The Cactaceae and the species of Mesembryanthemum are, of course, 
not the only plants which have paraheliode structures. Very few green 
land-plants are entirely devoid of such. A beautiful example of a plant 
living in dark caves, and therefore not provided with any paraheliode 
structure but rather with a light-collecting arrangement, is met with in the 
case of the protonema of Schistostega osmundacea. This plant is figured 
and described by Noll (20 and 24, p. 70). 
The epidermis which covers the upper and under surface of most 
leaves of our zone can, I think, be taken as having a paraheliode function. 
