PoiJits in Morphology mid Biology of Neiv Species of Haworthia. 393 
out by means of a few simple experiments. The assimilating tissue is 
represented by several layers of spongy parenchyma, in which at intervals, 
arranged in two rows, longitudinal sacs of rather long raphides of oxalate 
of lime can be seen. Adjoining the assimilating tissue are the vascular 
bundles in which sclerenchymatous elements are absent. The greater 
part of the interior is occupied by a thin-walled very transparent water- 
storing tissue. The whole structure resembles to such a remarkable 
degree Berger's figure * of a longitudinal section of H. retusa that at first 
sight I thought it represented our species. Both in this species and in 
ours very little light can enter the leaf except through the chlorophyll-free 
top, its "window." Similar arrangements, though somewhat modified, 
are found in many other species of Haiuorthia, H. retusa, and H. truncata 
being extreme cases. Now, in the description to his figure, Berger adduces 
H. retusa as an example of a structure in which the light is made use of 
to the greatest extent, while in the same figure he represents two other 
species covered with tubercles, which protect these species against too 
much light. It seems extraordinary that species of the same genus, 
provided they are living under similar conditions, should show such 
remarkable differences, some species being protected against excessive 
light, while others have special arrangements to make use of it. However, 
the apparent contradiction seems to be capable of an easy explanation. 
It is more and more recognised that plants cannot make use of intense 
sunlight ; in fact, their chloroplasts require protection against excessively 
intense light (compare, e.g., Schimper, " Plant Geography," Engl. Ed. 
p. 58). We have very little exact knowledge on this point yet, but the 
researches of Wiesner (see, e.g., J. Wiesner, ''Der faktische Lichtgenus der 
Pflanzen, Ber. d. d. hot. Ges., 1894, Generalversammlungsheft," p. 78), 
have shown that in the majority of plants the amount of light actually 
enjoyed is very much less than the amount available. Schimper points to 
the fact that the vegetation of very sunny spots is never pure green, 
and exhibits an admixture of yellow and brown tints due to the products 
of decomposition of chlorophyll,! but on the whole he does not think that 
land plants generally require very elaborate protection against excessive 
light, and yet an enormous number of our plants show almost at the first 
glance that only a small fraction of the sunlight which is at their disposal 
can be really utilised by them. Hairs, waxy covering, mutual shading of 
parts, succulency, and a host of other contrivances are to be found which 
must considerably reduce the light reaching the assimilating tissues. 
* Berger, I.e., p. 12, fig. 7, D. 
f Marloth, on p. 218 of his recently published magnificent work " Das Kapland, 
insonderheit das Keich der Kapflora, das Waldgebiet und die Karroo pflanzengeographisch 
dargestellts " (Jena, 1908), has incidentally emphasised this point with reference to many 
Karroo bushes. " There is no trace of green, even the scanty small leaves have the 
same nasty colour as the wood. Brown is the colour of the Karroo." 
