FORM OF THE PLANT-CELL. 



47 



is offered as an explanation of the facts, to guide farther inquiry, rather 

 than as a true law of development. There are two cases which seem to 

 form exceptions to this view. The first is the formation of pores, which 

 open into intercellular passages independent of the neighbouring cells. 

 These are very beautifully seen in the petioles of the Cycas revoluta. In 

 this case the single wall is easily affected, and the intercellular passages, 

 filled with air, act in the same way as the air-clefts. We frequently see, 

 also, a great air-cleft, forming a large fissure-like pore on one side, and 

 many little pores on the other side, as is frequently seen present in the 

 porous vessels of the Balsaminece. In a similar manner the porous cells 

 of the medullary rays in various species of Pinus often exhibit a longitu- 

 dinal air-cleft, which resembles the pores 

 in many cells. 



The last form worthy of mention is 

 when the cell-walls do not extend, and the 

 spires touch each other, but do not grow 

 together. This is the form on which 

 Meyen founded his false theory of the 

 fibrilliforin nature of all cell-membrane. 

 This phenomenon often presents itself, as 

 in the cells of the parenchyma of the 

 tubers of the Dahlia (Plate I. fig. 26.), 

 the hairs of the young leaves of Cycas re- 

 valuta, in the hairs of many Mammillarice 

 and Melocactece, the scales of the buds 

 of Pinus sylvestris, &c. Sometimes the 

 spires in these cases present fissures, as 

 is beautifully seen in the cells of the 

 rootlets of Oncidium altissimum (Plate I. 

 fig. 24.). 



Individual Development of the Spiral 

 Fibres, and of Abnormal Forms. Every 

 spiral fibre at its first visible existence is 

 a fine thread, which increases both in 

 breadth and length (fig. 31. c, d.). This 

 goes on so long as the cell contains sap, 

 but ceases immediately this is absorbed, 

 and the cells fill with air. In some cases a 

 part of the spiral fibre does not increase 

 with the rest, and the fibre terminates as 

 it were with a pointed end, as is often 

 seen in the vessels of the common gourd 

 (fig. 31. e.). Occasionally, and apparently 

 from disease, the cells which had originally 

 been filled with fluid, and which had 

 given place to air, are again filled 

 with fluid when a fresh set of anasto- 

 mosing spiral fibres are formed. This 



31 , Annular ducts from the stem of Canna occidentalis, with a regular distance of 

 the rings. 6, Annular ducts from the petiole of Musa sapientum, the vessel between 

 the two rings being distended, c, d, Spiral from a cactus, very young, and perfectly 

 developed, e, A spiral vessel from Cucurbita Pepo, with some of the spiral fibres ending 

 iu a point. 



