308 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



As tubercle and lateral root grow, the resemblances be- 

 tween the two decrease and finally disappear altogether. 

 The tubercle has no cap^ and no central strand of con- 

 ducting tissue. The tubercle cells differentiate into definite 

 tissues more slowly than do those of the lateral root, but 

 near the tip of the older tubercle there is a mass of meriste- 

 matic cells similar to the growing-point of a lateral root (see 

 fig. 7), This meristem forms cells forward and backward 

 as does the growing-point. The central mass of the tuber- 

 cle is proportionally much larger than the central cylinder 

 of the lateral root, but it is wholly undifferentiated. The 

 cortex of the tubercle contains vascular bundles, small and 

 separated from each other by considerable spaces of paren- 

 chyma (see fig. 7), and is enclosed by layers of cork-cells. 

 These may and usually do become powdery on the surface 

 and rub off as the tubercle forms, just as the cap cells do 

 from the tip of a root. 



In point of origin and in their earliest growth, tubercle 

 and lateral root are similar. In subsequent growth they are 

 more and more dissimilar. Morphologically, then, the root- 

 tubercles are lateral roots. Though called into activity by 

 very different causes, the cells of the pericycle give rise by 

 division to masses of cells which, on the one hand, develop 

 into tubercles, on the other develop into lateral roots. In 

 the one case we know the stimulus which causes the tuber- 

 cle to form. It is the infection of the root-cells down to 

 the innermost layer of the periblem by bacteria. Do lateral 

 roots form as the result of external stimuli or are they the 

 effects of causes internal to the plant? The latter is the less 

 likely from the fact that the size, number, and position of 

 lateral roots varies in plants of the same species according 

 to the soil, to the number and kind of other plants living in 

 the same soil, to the distribution of moisture and other 

 matters in the soil, and to a great number of other factors 

 not now recognized. This subject merits investigation. As 



1 According to Life (Botanical Gazette, April, 1901) the roots infected by Anabeena and 

 certain other organisms in certain species of Cycas also have no caps, yet he unhesitat- 

 ingly describes them as roots. 



