Januaky 1, 1896.] 



KNOWLEDGE, 



21 



the later stages of growth, its place is taken by cork 

 developed by the divisions of one of the outer layers of 

 cortical cells. 



Now, all these structures (with the exception of vessels 

 in the wood) can be seen in the stem of the mistletoe. As 

 a rule chlorophyll is present only in the outer cortical 

 layers of a stem and in certain cells of a leaf, but in the 

 mistletoe almost all the cells of the plant, stem, root, and 

 leaves, are filled with the green colouring matter. The 

 ramifications of the parasite in the tissues of the host are 

 thus easily detected. 



The haustorium, developed from the inner parts of the 

 radicle, passes through the cortex of the host-i^lant until it 

 arrives at the wood, which, however, it does not pierce. 

 But next year we find its apex buried in it ; the cambium 

 cells of the host have developed new wood, which has sur- 

 rounded the haustorial tissues. It is clear that the 

 continued production of fresh sylem year by year would 

 result in the destruction of the haustorium ; but provision 

 is made to obviate this. The cells situated near the apex 

 of the haustorium of the first year divide up in such a 

 maimer as to cause it to increase in length just as much 

 as the wood of the host increases in thickness, and simul- 

 taneously an increase in girth occurs, so that in a trans- 

 verse section of the host (Fig. 2) the sinker apj)ears wedge- 

 shaped. It is thus manifest that year by year the haus- 

 torium becomes more deeply embedded in, and tiius nlore 

 intimately fixed with, the wood of the plant on which it is 

 parasitic. 



The part of the haustorium in the cortex of the host is 

 composed of a central cylinder of numerous tracheides, 

 surrounded by thin-walled cells densely filled with proto- 

 plasms ; this is enclosed by a cortex of thin-walled cells 

 becoming thick-walled towards the periphery. The portion 

 of the sinker that has become embedded in the wood 

 possesses a large number of tracheides uniting directly at 

 its sides, but not at its tip, with the wood of the host. 

 Surrounding these are cambium-shaped cells. In a line 

 with the cambium of the host-plant are layers of cambial 

 cells, from which originate lateral branches. The parasite 

 is thus immediately connected, and very intimately, with 

 the wood only of its host. 



Now, this is just what might be expected. For the 

 possession of such an abundance of chlorophyll renders it 

 perfectly autonomous in the work of carbon assimilation. 

 It is thus quite independent of its host for the carbon and 

 oxygen necessary for the building up of protoplasm ; all it 

 requires from it are water and inorganic salts. These we 

 know, after their absorption from the soil by the root- 

 hairs, are carried up by the wood of the host. The 

 intimate fusion of its wood-elements with those of the 

 mistletoe ensures the passage of those inorganic substances 

 into the body of the parasite. 



The mistletoe is thus only a partial parasite. It merely 

 obtains the inorganic portion of its food-supply from its 

 host, and on it it appears to exert no baneful effect. The 

 host, nevertheless, cannot be nourished in turn by the 

 parasite, owing to the absence of sieve-tubes in, and the 

 presence of thick-walled cells at, the periphery of the 

 haustoria, bordered generally by dead phloem ceUs of the 

 host. 



There are other parasitical flowering plants whose elfects 

 upon their hosts are by no means of the harmless nature 

 of those of the mistletoe. The dodders (species of Cuseuta) 

 are examples of such. They are found in all parts of the 

 world. One of them {Cuseuta trifolii) attacks clover, and 

 its ravages at times are exceedingly destructive. It is said 

 to be a variety of Cusntta epilltymum that attacks thyme 

 and allied plants. Two other species are found in the 



British Isles — Cuseuta epilinum, which, as its name implies, 

 is parasitic upon flax {Luium), and Cuseuta Europcea, a 

 parasite on the nettle, hop, &c. The clover dodder spreads 

 over its host so as to leave a field of bright green clover a 

 mass of yellowish tangled thread-like stems. The seeds of 

 the dodder are doubtless sometimes sown along with the 

 clover seeds. In nature they fall from their seed capsules 

 in autumn, remain on the ground over winter, and ger- 

 minate the following spring, but a month later than the 

 majority of other plants. By the time the embryo emerges 

 from the seed the seedling clover has attained a fair size, 

 and plants of perennial growth have developed their young 

 shoots. The seed contains a mass of endosperm or 

 nourishing matter, which ministers to the nourishment 

 of the young plant during the early stages of its growth. 

 As in the case of other flowering plants, the root is the 

 first part to emerge from the seed-coat, but, unlike the 

 majority of these, it is unprovided with a root-cap, although 

 it is clothed with numerous short hairs. The root scarcely 

 enters the soil. Owing to its persistent parasitism, its 

 original structure has become much altered. Its central 

 axis no longer possesses vessels, as in other roots, to enable 

 the rapid conduction from the soil of water containing 

 nutritive salts, or sieve-tubes to convey elaborated sap to 

 growing parts ; but instead it consists of a cylinder of cells 

 densely filled with protoplasm, whose elongated form gives 

 a hint as to what w-ould occur were the plant to Uve again 

 an independent existence. 



The root enters the soil or creeps along the ground for a 

 short distance. The stem bends slightly above the sm-faco 

 of the soil, and circumnutation then takes place to enable 

 it to come in contact with a support. The moisture 

 absorbed by the root hairs, if the plant grows in a damp 

 atmosphere, is sufiicient to convert all the reserve material 

 in the seed into forms available for the nutrition of the 

 seedling, and the stem then grows apace and may attain a 

 length of a few centimetres ; but should the surroundings 

 .bo dry its growth is materially lessened, both in lenixth and 

 thickness. The apex of the sten; by-and-bye emerges from 

 the seedling, and it is seen to be devoid of cotyledons or 

 seed leaves. The root .soon dies away, and the food-material 

 contained in it is passed on to the growing part of the stem, 

 thus aiding it to further 

 increase in size. The 

 basal portion of the 

 stem also dies away, and 

 the food material there 

 is also conveyed to its 

 apex. Thus, should the 

 stem fail to come into 

 contact with a suitable 

 support before the death 

 of the root, there is 

 additional help given to 

 enable it so to do. The 

 direction taken by the 

 circumnutating stem is 

 from left to right, and 

 the first few spirals 

 formed arc very close. 

 This prevents the Cuseuta stem being thrust off the host 

 by its own rapid growth. 



The stem bears scale leaves only, and in their axils 

 arise flowers and branches. No chlorophyll, or. at 

 most, the merest traces are developed in the plant ; 

 consequently it must be wholly dependent on green 

 plants for its supply of food. Such is found to be the 

 case, ard the mode in which it obtains this we shall now 

 inquire into. 



Fig. 4. — Stein with Dodder fast«netl 

 to it : D, Roildcr stem ; f, its flowers ; 

 ir, stem of Uost» 



