BOTANICAL GAZETTE. 134 



are usually regarded as subsequent formations,* but may, however, 

 be the persistent partitions of cells that generally coalesce completely 

 to form single wood-cells. f The wall of the cells is differentiated into 

 three lamellae — a middle one, with one on either side — and has 

 simple pits not penetrating the middle lamella. These pits are twisted 

 in such a way that they have the appearance of being situated at the 

 intersection of the arms of an oblique cross, when seen in front view. 

 To determine their structure requires careful examination with a high 

 po.'er. Between the wood-ring and the epidermis lies a ring of tissues 

 of very considerable importance, but not homogeneous like the last. 

 It consists of parenchyma containing chlorophyll, in which lie numer- 

 ous masses of collenchyma in contact wuh the epidermis but not 

 extending quite deep enough to come in contact vvith the cortical 

 wood. The stomata are all situated in the part of the epidermis 

 touched by the chlorophyll bearing parenchyma, which is readily 

 distinguished upon the exterior of the stem as interrupted lines of 

 darker green. 



The fibro-vascular bundles are open, two-sided bundles, but 

 peculiar in having an additional phloem portion on the axial side. 

 The xylem and the outer phloem are separated by the cambium, in 

 which the ])rogressive transformation from simple uniform cells to the 

 various mature cells of each portion can be traced. Both the axial 

 and outer phloem consist of sieve-tubes, interspersed with long, 

 slender parenchyma cells, the two together forming "soft bast." 

 These are excellent examples of sieve tubes: the perforated end-parti- 

 tions, the broad tliin s| ots and sieve plates of the side walls, and the 

 conspicuous protoplasmic contents are readily made out in detail. The 

 xylem contains all gradations between the extreme form of annular 

 vessels with widely isolated rings, on the one hand, through spiral, 

 reticulated, scalariform, to pitted vessels, on the other hand. The 

 structure of the walls of these vessels can only be studied satisfactorily 

 under high powers. Between and about the vessels is wood- 

 parenchyma. 



To sum up the tissues of the stem of Cucurbita : 



Epidermal system : Fibro-vascular system : 



Epidermis. (Cambium). 



Stomata. Phloem. 



Hairs. Sieve-tubes. 



Fundamental system : Phloem parenchyma. 



Interfascicular parenchyma. Xylem. 



Hypoderma. Vessels. 



Cortical wood. Annular. 



Cortical parenchyma. Spiral. 



Collenchyma. Reticulated. 



Scalariform. 

 Pitted. 

 Wood parenchyma. 

 To these should doubtless be added laticiferous tissue sometimes 



^Saclis, Text-book. p. 101. 

 f Bessey, ISotanj-, p. 74. 



