230 
POPULAK SCIENCE PEYIEW. 
represent the difference between a small absorption and a great 
expiration. The expiration of carbon-dioxide is, of course, very 
exhausting to the host-plant, weakening it quite in the same 
way as a disproportionate and excessive production of its own 
flowers would do. 
It is also to be noted that parasitism is a condition which may 
vary in degree. The plants most absolutely dependent upon 
others are those which, like the Rafflesiacece, are quite vdthout 
provision for a separate existence ; the seeds, doubtless, though 
this has not been observed, germinate on the branches upon 
which the fully developed plants are found, as is the case with 
those of the various green parasites. The numerous leafless 
root parasites present several gradations from such absolutely 
parasitic forms as Gynomorium and the Balanophorce, through 
Orohanche, Harveya, Lathrcea, Striga, &c., to those ScropJm- 
lariacece and Santcdacece which are but slightly dependent for 
supplies of food on the plants to whose roots they are attached. 
Some of these forms are completely parasitic only in early life : 
this is probably the case with our Keottia (Bird’s-nest) and 
Monotropa. In that singular stem-parasite the Dodder, the 
embryo, entirely without cotyledons, is able to live for a short 
time after germination, and rises into the air until it attaches 
itself to some plant, when its connection with the ground is 
speedily broken. 
In attempting to classify the parasites all these conditions 
must be taken into account; but unfortunately the mode of 
attachment has not been carefully noted in numerous cases, 
whilst in several presumed parasites the actual union with 
another plant has never been observed. We may, however, at 
once divide them into the essentially distinct groups of chlo- 
rophyllous and non-chlorophyllous plants, and in classifying 
the latter we cannot do better than follow A. P. De Candolle, 
and group them into those with a single sucker (monobasic), 
those where the attachments are numerous (polystomal), and 
those where along with but one large sucker there are numerous 
thick fleshy roots (polyrrhizal). Further division is shown in 
the following table, though it must be admitted that the dis- 
tinctions between the first and third groups are not very well 
defined : — 
■^Without green leaves or chlo- 
ffophyll : 
Monobasic. 
On stems : 
Itaffiesiacece. 
On roots : 
Cytinacece. 
Balanophora, &c. 
Polystomal. 
On stems : 
Cuscuta. 
Cassytha. 
On roots : 
Lathrcea, &c. 
Cynomorium, &c. 
