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(Stem) Succulents and Geophytes

By Wayne Handlos, Ph.D.                                       All images by Wayne Handlos

1)      2)      3)       4)        5)

1) P. paniculatum  2) P. klinghardtense  3) P. klinghardtense 4) P. luridum  5)  P. incrassatum

Last month we looked at some of the diversity in the leaves of different species of Pelargonium. Many of

the leaf variations are adaptations to the climate of Southern Africa. In this article we will examine modification

of the stems and roots of Pelargonium species. Succulents are defined as plants which store water in specialized

storage organs. In some plants the leaves are specialized for water storage and these plants are referred to as

leaf succulents. Such plants often have waxy coverings and abundant hairs on their leaves and mucilage in their

cells. Ice plants, living rocks and many members of the Crassula family show these modifications. Other plants

 store their water in enlarged, fleshy stems and are called stem succulents. Most cactus plants and many Euphorbia

plants store water in this way. Various desert dwelling species of Pelargonium are stem succulents. Most of these

have stocky, thick stems with deciduous leaves. Pelargonium tetragonum is unusual in this group in having long,

thin but succulent stems.

Many of the species of Pelargonium grow in summer dry areas of southern Africa – in a Mediterranean climate

similar to ours in California – rainfall in winter (the cold season) and a dry, warmer (or even hot) summer. A number

 of species especially in section (§) Otidia have thickened stems capable of storing quantities of water in the cells: a

stem succulent. In the case of P. klinghardtense the leaves are also somewhat succulent (fleshy) with stored water.

All the other species in this section have succulent stems and most have leaves described as fleshy or succulent

Species with succulent stems are also found in the sections Cortusina (all except P. sibthorpifolium), Isopetalum,

Jenkinsonia (P. tetragonum), Ligularia (P. crassipes, P. fulgidum, P. hirtum, P. hystrix, P. karooicum, P. oreophilum,

P. pulchellum), Peristera (P. drummondii), Polyactium (P. gibbosum, P. apiifolium, P. vasseri). In the section § Ciconium,

P. zonale, P. inquinans (the two ancestors of zonal geraniums) and P. quinquelobatum have somewhat succulent/fleshy stems

 (which makes them tolerant specimens of our cultivation techniques) although they are not usually considered as stem succulents.

Geophytes are a category of plants that survive unfavorable growing conditions with their buds underground. In the case

of the species of Pelargonium they survive the dry season as tuberous roots, tap roots or occasionally as rhizomes. In

cultural terms most of these plants undergo a period of dormancy during the dry season – so in cultivation they should be

kept dry during dormancy. Many tend to rot if wet during their dormant period. The § Hoarea – named for Richard Colt

Hoare, a grower and hybridizer of Geraniaceae in the early 19th century – is characterized by the presence of tuberous

roots covered by thin papery bark (periderm) and bearing a tuft of dead leaves and stipules at its apex. Tuberous roots

are found in about 20 species in about half of the other sections of Pelargonium though none of these sections is characterized

by the presence of tubers. Such commonly grown species as P. echinatum, P. carnosum, P. caffrum, P. lobatum, and P.triste

have tuberous roots of some form.

The Danish ecologist Raunkiaer defined a number of plant life forms based on the position of their “perennating” organs

(buds) including the category geophyte. Geophytes have their dormant parts underground in dry ground.

Africa has many plants which develop underground storage organs and are appropriately called geophytes. They form a

veritable cornucopia of botanical structures popularly called “bulbs”. But in a botanical dictionary we would distinguish them

as distinct types based on their individual structures. The underground storage organs include tuberous roots, taproots, corms,

stem tubers, rhizomes, pseudobulbs, caudex, storage hypocotyls, and bulbs.

          6)     7)    8)    9)

6) Image 6 shows the rows of cells in the cork (periderm) of the stem of P. carnosum. New cells are produced

 by the thin-walled cells and older cells are pushed outward by the newer cells.

7) This image shows the water storage cells at the center of the stem of P. tetragonum. The oval structures are starch grains –

 a common food storage structure in plants.

8) Cross section of a portion of the stem of P. tetragonum. The outermost cells contain abundant green chloroplasts where

the bulk of photo-synthesis (food making) occurs. A layer of cells with thickened walls (the gray layer) provides structural

support to the stem. The large cells are the food (starch grains) and water storage tissue at the center of the stem.

9)  3-D crystal (druse) in a stem cell of P. alternans.

 

© 2015, Central Coast Geranium Society (CCGS )