36 PRACTICAL BOTANY 



host are total parasites, while the others are partial parasites. 

 The dodders (Fig. 351) are practically leafless, of a yellow- 

 greenish or whitish color, and incapable of photosynthesis. 

 The mistletoes and many other half parasites have green 

 leaves and can do photosynthetic work, so that they may 

 depend on the host only for water and mineral subtances, but 

 make for themselves starch or other carbohydrates from the 

 raw materials. Root parasites (Fig. 309) are often attached 

 to the roots of the host at some distance from the stem of 

 the latter, so that few but botanists recognize the real state 

 of the relations between the two plants. The sucking roots 

 of parasites, known as haustoria, are of peculiar structure and 

 have the power of penetrating rapidly into the substance of 

 the host. In the dodder, at any rate, this power is partly due 

 to the presence of ferments, liquid or semi-liquid substances 

 manufactured by the haustorium and capable of dissolving 

 cellulose. 



37. Partnership of roots and bacteria. Bacteria are exceed- 

 ingly minute plants of very low organization (Fig. 150). 

 Their forms and structure are shown in a general way by the 

 figure. They differ greatly in their habits of life, as is shown 

 in Chapter XL Those which inhabit little tubercles on the 

 roots of most leguminous plants (as those of the Pea family 

 are called) are of the highest value to the farmer. The tu- 

 bercles occur in the greatest abundance, 4572 having been 

 counted on the roots of a single pea plant. Fig. 305 shows 

 their mode of occurrence on the roots of red clover. Each 

 tubercle contains multitudes of root tubercle bacteria, which 

 are able to change the free nitrogen of the air, contained 

 in the pores of the soil, into a combined form in which it can 

 be absorbed by the plant. Without the action of these or 

 other bacteria or other agents to transform atmospheric nitro- 

 gen into a soluble form it is perfectly useless to the higher 

 plants. 1 



1 It is certain that other bacteria besides those of root tubercles render 

 nitrogen available, but the extent of their action is not fully known. 



