ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
205 
been made to ascertain the temperature of the tubercles, it has been 
determined that the point of maximum temperature is when the aerial 
stems commence to appear. 
The anatomy of the aerial stem and that of the tubercle differ 
greatly. If the structure of the underground stem bearing the tubercle 
be examined, it will be found to be quadrangular, and at each of the 
angles a layer of collenchyme protecting a fibrovascular bundle will be 
seen ; then a cortex fifteen to eighteen cells in thickness, and then the 
pith. If a transverse section be made of the first slightly swollen 
internode, the collenchyme will be found to be diminished, the cortex of 
about thirty cells in thickness, the wood diminished in quantity, and the 
jiith greatly augmented. If a section be made at the base of the aerial 
stem, the collenchyme will be found greatly developed, and protecting 
four large fibrovascular bundles, a diminished cortex, a zone of pericyclic 
lignified fibres, and finally a greatly diminished pith. The tubercle is 
then formed by a considerable development of the pith accompanied by 
a relatively less augmentation of the cortex. 
M. P. Maury* states that the morphological value of the tubercles 
of Stacliys affinis Bge. (^S. tuherifera Naud.) is the same as that of the 
potato, both being swollen subterranean stems. If a transverse section 
be made of an internode towards the middle, the following will be the 
arrangement from the periphery to the centre : — in the first place an 
epiderm, on the exterior of which is a thin layer of cutin ; the cortex, 
formed of large roundish cells ; then the fibrovascular zone, consisting 
of four principal fibrovascular bundles ; and finally a very bulky pith. 
The difference between the structure of a tubercle and that of an aerial 
stem is but slight, and is principally marked by the absence of stomates 
and chlorophyll and the predominance of pith in the tubercle. 
Non-chlorophyllous Humus-plants. | — Herr F. Johow describes the 
peculiarities of structure of the “ holosaprophytes,” or saprophytes 
destitute of chlorophyll, of which he enumerates 165 species belonging 
to 43 genera and 5 natural orders, viz. Orcbidacese, Burmanniacere, 
Triuriacem, Ericacea^,, and Gentianacese, all the species of the two genera 
of Triuriacea3 belonging to this class of plants. Of these about 44 
belong to Temperate, and 121 to Tropical couutries. Most grow in the 
soil, some on rotten branches of trees, SciapMla (Triuriacese) on the 
nests of termites. 
The roots are generally but feebly developed, and well-developed 
root-hairs entirely wanting, except in Sciapliila. The central cylinder 
exhibits varying peculiarities of development. Except in Wullschlsegelia 
(Orchidacete) the roots are always invested by a mycorhiza, which 
(except in Monotropa Hypopitijs) does not penetrate beyond the epidermal 
cells, and affects the cells so little that they still contain protoplasm 
and even a nucleus. It is apparently this fungus that causes the usually 
coral-like or tufted appearance of the root. The author believes that 
the mycorhiza absorbs not only nitrogen, but also the nutrient substances 
resulting from the decay of the humus. In epiphytic orchids the 
mycorhiza is wanting in those parts which hang free. Except in 
* T. 0., pp. 186-9. 
t Jahrb. f. Wiss. Bot. (Pringsheim), xx. ^889) pp. 475-525. 
