432 SCIENCE PROGRESS 



water which is taken up by the roots and from the carbonic 

 acid gas absorbed from the air by the leaves and green parts 

 of the plants. Under favourable circumstances, after translo- 

 cation from the roots to the shoots, the raw materials in 

 solution undergo certain chemical changes and are elaborated 

 into substances which are capable of being used to nourish 

 the plants. To effect assimilation two factors are essential, 

 the presence of a sufficiently strong light in conjunction with 

 chlorophyll, the green colouring matter of plants. The parasites 

 as a class are either destitute of chlorophyll or contain very 

 little of it but by association with other species of plants, known 

 as " hosts," are able to derive their nutriment by specially 

 devised means of robbery. Three of the commoner weed 

 parasites are (i). Dodder (Cuscuta trifolii), (2) Yellow Rattle 

 {Rhinanthus crista-galli), (3) Broomrape {OrobancJie minor). 



1. Dodder. — This plant, which is closely related to the Bind- 

 weed, is of annual duration and as an agricultural weed is 

 chiefly parasitic on clover and similar leguminous plants. The 

 seed germinates in the soil and produces an undifferentiated 

 threadlike seedling, which pushes up into the air and twists 

 about until it finds a living plant or host to which it can attach 

 itself; the Dodder twines round the leaves and stem of its host 

 and finally, when attached, dies off itself at the base, so that 

 all connection with the soil is lost. Numerous small suckers or 

 "haustoria" are sent out which penetrate the tissues of the 

 attacked plant and act as sucking organs by means of which 

 elaborated food material is transferred from the host to the 

 parasite. The Dodder is reddish or yellow in colour, never 

 green, as no chlorophyll is formed and no leaves are produced ; 

 the long thin straggly stems develop an abundance of dense 

 clusters of small whitish flowers, which are succeeded by 

 rounded brown seed-pods. Large quantities of seeds are 

 produced, so that the absence of Dodder seed from any crop 

 seed which is intended for sowing is essential if a clean crop 

 is to be obtained. If the Dodder secure a good hold it kills its 

 host plant by strangling it and by robbing it of food, thus 

 causing it to starve. It spreads rapidly, throwing its straggly 

 stems from plant to plant ; if unchecked, it may easily cover 

 a whole field of clover and kill it entirely. 



2. Yellozv Rattle. — In contrast to the last this is only a semi- 

 parasite in that it does bear green leaves but apparently the 



