PHYSIOLOGY. 



261 



arises from the appearance of dots which 

 present. 



Fig. 231 represents the modification of 

 the spiral, into the punctuated vessel. The 

 dots are exceedingly minute, but greatly 

 magnified in the figure. They surround 

 the vessel in parallel rows, the fibres of the 

 spiral being separated, and the intervening 

 space continued by a fine connecting mem- 

 brane. This membrane is not present in 

 the young plant, and does not form until 

 the leaves are of full size. In the young 

 plant it is transparent, but becomes opaque 

 by age. It is the largest in respect to di- 

 ameter of any of the vegetable vessels. 



Beaded Vessel This is a modification of the punctuated, 

 and reticulated vessels. It resembles a chnin of oblong ovate 

 cells, or beads, whence its name. Moniliform, by which 

 name it is sometimes called, means " like a necklace." 



Fig. 232, shows the form of this vessel. It is F '- 232 - 

 found only in the knots of the stem, "and the tuber- 

 cles of the roots, and seems to be designed for the 

 union of the other vessels, with each other. 



Sometimes two or more modifications of structure 

 are seen at the same time, and in the same tube. 

 This arises from the natural tendency of the simple 

 spiral to change into the punctuated vessel, and both 

 this, and the reticulated, to change into the beaded 

 vessel. If the stem of a Gourd be examined at different 

 periods of its growth, or in different parts of the plant 

 at the same time, between the root and the top of the stem, 

 it will be found, that in the young plant, or the newly unfolded 

 extremities of the old one, the vessels are all simple spirals. 

 As the plant advances in age, each of those already formed 

 become punctuated vessels, while new spirals are forming in 

 the growing parts ; and this transformation of spiral into 

 punctuated vessels, proceeds until all the vessels in the older 

 parts of the stem, that are near the root, become punctuated. 



RETURNING VESSELS. It has already been stated, that 

 as soon as the leaves begin to appear, the sap ceases to flow 

 through the wood. The bark at this time is full of fluid, 

 which is descending towards the root. The vessels through 



