Page:Popular Science Monthly Volume 25.djvu/664

648 force their way through the bark, penetrate the tissue, and take the matter found there into their own systems.

Still closer examination will reveal other features. In the first place, there are none of the green leaves usually found on plants. Secondly, there is no root fastening the plant to the soil. Why is this? What is the reason that this plant grows and flourishes like other plants, and has yet neither root nor leaves? Let us see.

What is known as parasitism in plants is not confined to any one family or class. Various orders have one or more genera with species which take their nourishment in a complete or partly elaborated form from other plants. Sometimes they are perfect parasites, and take everything they need from other vegetable forms. Sometimes, as in the mistletoe, they take the partially made sap, and complete its transformation within their own tissue; while in still other instances only a very little of the sap is taken, and the other nourishment is absorbed from the soil by the roots proper.

Our dodder is an example of a perfect parasite. All the material necessary for its growth it takes ready made from the plants upon which it grows. As the purpose of leaves in all plants is to prepare from the crude materials in the air and soil the matter necessary for its growth, and as the dodder finds and appropriates this material already made, the absence of leaves is at once accounted for. There was no need for them, and they ceased to be.

The want of a root is another matter. When the seed of the dodder is examined, it is found that there is simply a coiled embryo, with very little albumen. The usual seed-leaves are absent; so that, for its first growth, it must depend entirely upon the albumen in the seed. When this seed first germinates, a little rootlet penetrates the ground. Owing to the deficiency of food, it only exists long enough to enable its stem to grow till it reaches some plant upon which it can fasten. When this is accomplished, the young plant will grow rapidly, and soon sever its connection with the ground; but, if not able to reach some support, it dies entirely.

In order to comprehend the reasons for the peculiarities of the dodder, and understand how it came to assume its habit of complete parasitism, it will be necessary to notice the probable rise and progress of the habit. We can do this by looking at some of those plants which are not yet such complete pensioners on the bounty of others. For it very seldom happens that all the steps leading from a normal to an out-of-the-way mode of living are lost. Some few will remain, to indicate the line along which the plant has proceeded. Imperfect adaptations point surely the path leading to perfect development.

The modes of living of the dodder and the Indian pipe may be considered as the two extremes of one line of development. The first is a complete parasite, and the second has gone so far as to become a saprophyte. The central point from which sprang the two branches