Zea mays

2.68
2.67 Development of II sieve tube and its
companion cells in a flowering plant. A,
sic:ve: dement precursor cell; nOle its thin
primary wall (1), large ,entrlll vacuole (2)
and two nudei (3) with II de\"e!oping cell
plate (4) between them. E, mature sieve
clement and companion cells. The largest
cdl formed from the precursor gives rise to
the sieve: dement; which is enucleate: but
Slill Terains its plasmalemma, modified
plastids (5), miwchondri3, endoplasmic
reticulum and proteinaceous fibrils (6).
The end walls of the clement now form
the sie\'c plates and are perforated by pores
which are often apparendy occluded by
fibrils. The cell wall is thickened at the sieve
plale, and around the pores it comains
amorphous callose. The [wo densely q'toplasmic,
nucleated companion cells are
formed after the funher division of the
smaller derivative of the precursor cell
shown in A. Companion cdl nucleus (7).
I Primary wall
2 Central \'acuole
3 Nuclei
4 Developing cell plate
5 Plastid
6 PrOteinaceous fibrils
7 Companion cdl nuclei
2.68 Sieve tube from the S[em of U,lum I/sitatissimum
(flax}. Note the obliquely inclined sie\'e plate with whirish
regions of callose surrounding rhe sieve pores (I). Several
plastids (2) containing starch are visible in rhe sieve rube:
and numerous fibrils of P.protein fill irs lumen. Companion
cdl3. (G-Os, TE.\1 x 17,000.)
I 1 Sieve pores
2 Plasrids
3 Companion cell
,-------------------, 2.67
A
B
• 'f--1
rt--4
2.69 TS of secondary phloem (formed from a seeondary
thickening meristem) frOIll the stem of the monocotyledon,
Dracaena. Note the oblique sieve pbtes wirh c:lllose (white
:lreas) around the pores. Pbsrids (1) with st:ln.:h gmins :lre
present in the lumina but P-protein is absent. (G-Os, TE\.l
x 6,000.)
Q P13_,_"_"' _
55
2.69