120 TECHNICAL BULLETIN 4. 



points upward. At this early date the point of division between radicle 

 and cotyledon is indicated by a slight swelling, the root joint, rj. As all 

 the parts continue to elongate rapidly the curve in the cotyledon becomes 

 a sharp knee, kn , the part between the knee and seed is the ascending leg, 

 al, while that between the knee and the root joint is the descending leg, 

 dl. The primary root grows down very rapidly and is soon several times 

 as long as the cotyledon. From about the fifth day it will be noted that 

 the descending leg elongates more rapidly than the ascending leg. The 

 first part to appear above ground (seventh to tenth day) is the tip of the 

 knee, and each part becomes green as soon as it has reached the light. 

 The seed may still remain in the ground for a week or more after the knee 

 has appeared, but since it is firmly attached, and since the descending leg 

 continues to elongate more rapidly than the ascending leg, the seed is 

 finally carried into the air (Fig. 3, C). The knee then has a tendency to 

 straighten out, but its position is indicated as long as the cotyledon lives 

 by a sharp kink. On about the ninth or tenth day the first secondary root, 

 r2, may be seen pushing out from the swollen root joint, and this is fol- 

 lowed later by others in rapid succession, rS. Meanwhile the first leaf 

 bud has been elongating rapidly. The cotjdedonary cavity elongates also 

 in proportion (Fig. 3, D). It should not be understood that this cavity 

 is absolutely included, without any opening to the outside; on the con- 

 trary, its upper narrowed apex communicates with the outside air through 

 a small longitudinal slit in the side of the cotyledon (CO in Fig. 3, C and 

 D). As the leaf bud pushes its way upward the sides of the cavity are 

 distended, and finally from about the seventeenth to twenty-fifth day the 

 tip passes through the slit and appears on the outside as the first leaf 

 {lb in Fig. 3, C). But before this time the primordium of the second leaf, 

 lbs, has appeared in a depression at the base of the first, and successive 

 leaves follow rapidly, each starting from the base of the next preceding 

 at a very early stage. The successive secondary roots also start from the 

 same region. This very active meristematic region, the growing point, gp, 

 is very restricted, and remains stationary in the onion until after the bulb 

 is formed. The limited size and stationary position of the growing point 

 from which all new organs, roots or leaves, originate are characters of 

 prime importance in the spread of the smut fungus within the host plant. 



Period of Susceptibility. 

 It is a well-known fact that onions are susceptible onlj^ in the seedling- 

 stage, and are immune after a certain stage of maturity is reached. But 

 we have no exact knowledge of the duration of this period of susceptibility, 

 the exact stage or time at which infection first occurs, or the stage or 

 time at which it ceaocs. The establishment of two points is thus nec- 

 essary: (1) the first day on which infection takes place, and (2) the last 

 day during which the plant can be infected. The latter of these two points 

 was established by the following experiment. Seed was planted in a flat 

 of sterilized soil. Beginning with the third day, when the radicle on the 



