CHAP. L] by Malpighi and Grew. 241 



of the structure of the root and stem that a beginner may still 

 use them with advantage ; such figures as those on plates "36 

 and 40 and elsewhere show that he knew how to fashion his 

 observations by aid of much reflection into a clear representa- 

 tion of the thing seen ; there are, as might be expected, many 

 errors in the details of the more delicate structure of the 

 various forms of vessels and cells. 



Malpighi had not said, whether he considered the cells of 

 the parenchyma (the. term parenchyma comes from Grew) to 

 be perfectly closed or porous, nor how they cohere ; Grew 

 leaves no doubt on this point; he says distinctly on page 61 

 that the cells or vesicles of the parenchyma are closed, that 

 their walls are not traversed by any visible pores, so that the 

 parenchyma may be compared to the foam of beer. He 

 quotes Malpighi's view respecting the vessels of the wood, and 

 supplements it by saying that the spiral band is not always 

 single, but that two or more bands entirely separate from one 

 another may form the wall of the vessel, and also that the 

 spiral thread is not flat but roundish like a wire, and its turns 

 are more or less close together according to the part of the 

 plant. He also notices that the spiral tubes are never 

 branched, and that when they run straight, as in Arundo 

 Donax, they can be seen throughout considerable distances. 

 The view of the structure of spiral vessels, which began with 

 Malpighi and was maintained through the whole of the 

 1 8th century, Grew (p. 117) expresses still more distinctly 

 than Malpighi ; but it is to be observed that neither of them 

 clearly distinguished true spiral vessels with separable spiral 

 threads from vessels of the kind which occurs in secondary 

 wood, and only shows a spiral structure on being torn. 

 From the way, says Grew, in which the threads are woven, 

 it comes to pass that the vessels often unroll into a flat 

 surface, as we may imagine a narrow ribbon wound in a 

 spiral about a round staff so that edge meets edge ; and if the 

 staff is drawn out, the ribbon so wound will remain behind 

 R 



