ORGANS OF NUTRITION. 



Fig. 179. 



91 



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J i t: I i a. 



Fig. 179. Diagram of a Monocotyledonous stem. A. Transverse section. 

 B. Vertical section, a, a. Parenchymatous tissue. 6, 6. Pitted vessels, 

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



original separate bundles into a solid hardened mass resembling 

 wood. 



The structure of the vascular bundles thus dispersed in the 

 cellular substance of the stem, we have already described under 

 the head of Definite Vascular Bundles (see page 72 and fig. 167). 

 It was formerly supposed that these bundles, as they were suc- 

 cessively developed, were directed towards the centre of the 

 stem, and continued their course in the same direction towards 

 its base (as seen in fig. 180, a, b,c, d), the last-formed bundles 

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

 cumference those which had previously been developed. Hence 

 the name endogenous or inside growers, by Avhich these stems 

 are commonly known. The researches of Mohl first showed 

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

 the following is that Avhich really takes place : — The vascular 

 bundles have their origin in the punctum vegettitionis of the stem, 

 and are fully developed with its growth upwards and outwards 

 into the leaves, and downwards and outwards towards the 

 circumference. In other words, to render it more simple, the 



