PARASITIC AND SAPROPHYTIC PLANTS. 



41 



Degradations in the Structure and Functions of Parasites. — The most 

 obvious sign of degeneracy in many parasites is the want of chlorophyll. 

 They might be grouped artificially into chlorophyllous and non-chloro- 

 phyllous. Such represent different stages in degrees of degeneracy ; for 

 although the presence of green leaves in the group EivphrasiecB, such as 

 Rhinanthus, Bartsia, Euphrasia, Melampyrum, &c., would lead one to 

 conclude on a priori grounds that they could assimilate carbonic acid just 

 as ordinary flowering plants, yet M. Gr. Bonnier has discovered that the 

 above-mentioned genera, excepting the last, which in other respects differs 

 somewhat from its allies, do not disengage oxygen in light, whatever be 

 its brightness or the method of investigation employed. The inter- 

 pretation of this remarkable fact is that the assimilative power is so 

 enfeebled that the oxygen emitted is all reabsorbed for respiration, there 

 being no excess to be given off as in the case of all non-parasitic flowering 

 plants. 



Great degeneracy of structure is seen in both the vegetative and repro- 

 ductive systems of parasites, and also of saprophytes. Besides the total 

 absence of chlorophyll, the leaves of such parasites which have none are 

 reduced to scales or are absent altogether. The microscopic details follow 

 suit in the general absence of stomata, &c., while the fibro-vascular 

 cylinder of the stem exhibits various anomalies, reminding one of the 

 structures in stems of climbing plants and Monocotyledons. The great 

 dislocation of the cords is a result of parasitism, just as it is in climbing 

 stems, and is also a result of an aquatic habit, which has set up the well- 

 known arrangement in Monocotyledons.* In the case of the flowers of 

 parasites a very common feature of degeneracy is seen in the ovules, &c. 

 For, while a normal ovule, say of a buttercup, would consist of a central 

 nucellus containing the embryo sac within it, and surrounded by two 

 coats, in some parasites, as the mistletoe, there is nothing but a naked 

 embryo sac. When an ovule becomes a seed, the embryo, instead of 

 having two cotyledons a plumule and a radicle, may remain arrested in 

 the pro-embryonic condition of a globular cellular body, the outermost 

 skin of the seed being a simple cellular sac. Such seeds, for example, 

 occur in Orohanche, Bafflesia, Balanophora, &c. This kind of degeneracy 

 in the reproductive system seems to be one of the first conditions, for it 

 occurs in plants unsuspected of parasitism on other grounds, from which 

 true parasites have descended, as, e.g. Pyrola rotundifolia, from which 

 genus the greenless saprophyte Monotropa has probably been derived. 

 Again, the orders Loranthacece, containing the mistletoe, and Santalacece, 

 the parasitic genus Thesitim and others are allied to Olacacece, in which 

 the embryo is reduced to a nucleus, the coats being undifferentiated. But 

 this order does not contain known parasites. 



Parasites assisted by an Insectivorous Habit. — Parasites, and sapro- 

 phytes as well, are of course at a great disadvantage in having to depend 

 upon other plants, &c. for existence, for unless a seedling can soon attach 

 itself to something it must die. Two cases are known in which nature 

 has come to the assistance of the parasites, by furnishing them with the 

 means of catching insects from which they can derive some nitrogenous 



* Origin of Plant Structures, pp. 206, 214, 220 and 149, 178. 



