ORGANS OF NTJTEITION, 



85 



name of definite or closed vascular bundles (page 67, and fig. 159); 

 but we have still to describe their origin and direction through the 



Fig. 171. Diagram of a Monncotyledonous stem. A. Transverse section. 

 B. Vertical section, a, a. Parenchymatous tissue, b, b. Pitted vessels, 

 c, c. Woody fibres or cells, d, d. Spiral vessels. Alter Carpenter. 



stem. It was formerly supposed that these bundles, as they were 

 successively developed, were directed towards the centre of the 

 stem, and continued their course in the same direction towards 

 its base as seen in fig. 172, a, h, c,d, the last-formed bundles 

 being the most internal, and gradually pushing towards the cir- 

 cumference those which had previously been deposited. Hence 

 the origin of the name endogenous or inside growers, applied 

 to these stems. The researches of Mohl first showed that the 

 above mode of growth was not strictly correct, but that the fol- 

 lowing is that which really takes place : — The vascular bundles 

 have their origin in X\ie punctum vegetatio7iis of the stem, and 

 are fully developed with its growth upwards and outwards into 

 the leaves, and downwards and outwards towards the circum- 

 ference. In other words, to render it more simple, the bundles 

 may be traced to the leaves, from which organs they are at first 

 directed towards the interior of the stem {fig. 173, a, b, c, d), 

 along which they descend generally for some distance, and then 

 gradually curve outwards again and terminate at the circum- 



