ECOLOGICAL i8i 



(luite different from the close coils, which are like those of a tendril. 

 There is usually an alternation of close coils and loose coils. 



Since the root of the seedling is a transient vestige, the stem- 

 suckers of the dodder absorb soil-water from the sap-wood elements 

 of the host. Since the foliage of the dodder is represented only by 

 minute scales, and since chlorophyll is wholly or almost wholly 

 absent, there can be little or no photosynthesis, and there is an 

 absorption of carbohydrates (especially glucose) from the host; and 

 probably of proteins as well, for the "sieve-tubes" of the two plants 

 are in contact. Cross-sections show that the tissues of the parasite 

 come into intimate union with similar tissues of the host, wood with 

 wood, and bast with bast. Indeed, it is sometimes difficult to detect 

 the boundary-line. The suckers produce ferments which can digest 

 not only the starch, but the cell-walls of the host. 



Peirce has noticed that if a dodder plant be prevented from 

 forming close coils and suckers, it may develop an appreciable 

 quantity of chlorophyll — enough to keep it alive! The same slight 

 greening has been observed in dodders grown in the shade or on 

 weakened hosts. Another interesting experimental fact is that 

 Cuscuta monogyna has been grown to maturity without a host at 

 all, in a culture of glucose solution. 



The habits of dodders are repeated in tropical countries by 

 species of Cassytha, but these belong to the Laurels, not to the 

 Bindweeds — an interesting evidence of the independent origin of 

 the same adaptations in unrelated plants. In the Cassythas there is 

 the same twofold absorption of watery and elaborated sap ; and they 

 also illustrate like dodder a change from yellow to greenish when 

 grown in the shade. 



Root-parasites. — About the base of broom-plants and whins we 

 occasionally find a remarkable parasite — the broom-rape (Oro- 

 banche). A brownish fleshy stem stands upright for a foot or more, 

 bearing scales and numerous dull-coloured fiowers. There is no 

 chlorophyll, and the organic food is obtained by sending numerous 

 suckers into the roots of the broom. There is a very intimate union 

 of the root-tissues of parasite and host. An interesting feature, also 

 illustrated by some other root-parasites, is that the wind-borne 

 seeds, which are very numerous and very minute, will not germinate 

 except in actual contact with the roots of an appropriate plant. As 

 some kinds of Orobanche are restricted to one host, and none have 

 many alternatives, we can understand that variations in the direction 

 of abundant and minute seeds would be favoured in the course of 

 Natural Selection. 



Another striking root-parasite is the Toothwort {Lathrcea squa- 

 maria), at home on the hazel. The underground stem is covered with 

 fleshy tooth-like leaves, each bent on itself so as to form a cavity 

 with a narrow slit Tike opening. In this cavity there are numerous 



