Previous Thesis on the Areole Development and Their Discussion

The areoles of Cactaceae, sometimes referred to as "spine-bearing cushion", correspond to a shortened lateral shoot with leaves transformed into spines and the disposition of an axillary bud ("sleeping eye") which may produce either a flower or a shoot. The long-living meristematic region, from which flowers or shoots may develop, is reduced to the adaxial (upper) end of the areole in many Cac-taceae.

However, in some genera, among them Coryphantha, the areole is elongated in such a way that the insertion of the spine bundle represents a small, clearly demarcated appendage of the whole areole.We often refer to this insertion of the spine bundle as "areole", because a specific expression for the whole areole does not exist, although an appropriate one would be podarium (Zimmerman). In cactus literature the opinion prevails that in certain genera a serial division of the meristem occurs with production

Fig. 30. Interpretation of the Mammillaria phylad (from A.D. ZIMMERMAN 1985, Fig. 8). Details of the Mammil-laria phylad. The treatment of Ferocactus is very crude for lack of data from this genus. Numbered apomorphies are as follows: 1 foveolate seeds; 2 adult stems remaining tuberculate (neoteny); 3 loss of scale-like bracts from peri-carpel; 4, 4a, 4b indehiscent (and mostly juicy) fruits; 5, 5a suppression of areolar glands; 6 two-parted areoles (Mammillaria type); 7,7a, 7b loss of perisperm; 8 (and 8a in Coryphantha recurvata) subapical flowering; 9,9a, 9b, 9c, 9d, 9e bright red fruits; 10,10a reddish brown seeds; 10b reversal: secondarily black seeds; 11 strongly sinuate anticlinal walls of testa cells; 12 unique hypodermis in Mammillaria spp.; 13 Latex ducts; 14 sporadic presence of areolar grooves on podaria of adults; 15 Protomammillaria-type areole sequence; 16,16a, 16b, 16c Escobaria-type areole sequence; 17,17a, 17b, 17c smooth or weakly reticulate seeds; 18 gigantism; 18a reversal: greatly reduced stem size in Stenocactus; 19,19a, 19b, 19c, 19d multiple hypodermis with glands. Note that this hypothesis would, if correct, imply the re-appearance of perisperm in the most highly derived Mammillaria taxa

of an areolar meristem on the one hand, which is transferred on to the tip of the areole by growth and which has the capacity to produce spines, and on the other hand, the production of an axillary meristem which has the potential to produce flowers. This theory was presented by WETTERWALD 1888, and Buxbaum and most other botanists have followed him. Buxbaum in KRAINZ (19561960) described the serial division as follows: "...In highly derived Cereoideae the top of the axillary shoot is shifted in such a way that between the spine-bearing part and the flower- or shoot-producing part a groove develops, which extends on the upper surface along the tubercle ("Coryphantha stage"). Ultimately, this serial division is completed in such an early stage of development that an abaxial, areole-forming part and an axial, flower-bearing part, the axil, are separated from the beginning, therefore, no groove develops ("Mammillaria stage")."

Following this interpretation by Wetterwald and Buxbaum, which postulates a serial division, it remains unclear why, during this meristematic division into an outer (areolar)

vegetative part and an inner (axillary) fertile point, the groove formation of Coryphantha should take place in an axillopetal direction, i.e. from the spiniferous areole to the axil. This direction of groove development can clearly be verified in species with Escobaria areole type, but also with Macromeris and Protocoryphantha areole types (C. poselgeri-ana, C. robustispina). The development of the Protomammillaria areoletype in which grooves develop after activation of both the areolar and the axillary meristem also remains unexplained, as does the development of the Ortegocactus areoletype with optional groove formation. Postulation of a division of the meristem completely neglects the appearance of extrafloral nectary glands (morphologically honey spines) in the grooves and their continued secretion until the death of the tubercle. The same is true with regard to the trichomes (woolly hairs) in the grooves.

Two more facts remained unnoticed: 1. The spines of Coryphanthas continue to grow after the appearance of flowers and after the tubercles grow from the top of the plant. 2. New shoots arise exclusively from the groove near the spiniferous areole.

A somewhat different interpretation of the development of axils in Mammillaria was given by N.H. BOKE (1953) in his very profound histological studies: the axillary meristem develops from the general peripheral meristem after extinction of the areolar meristem, which has moved to the tubercle tip. According to Boke, there is no serial division of the meristem, but a spontaneous formation of buds in a series, as is usually the case for all dicotyledons.

A thesis of areole development which, contrary to the general opinion, does not proceed from a serial division of the meristem and which gives a simple explanation for the axillopetal (from spiniferous areole to axil) direction of the groove development was published in 1968 by Heimo FRIEDRICH: In his opinion, a separation of the areole meristem does not occur in Coryphantha, nor is it the result of such a division present in Mam-millaria. He postulates that the axillary shoot grows ventrally along the whole length, thus its base immediately follows the rudimentary leaf, and its tip is fixed in the axillary cortex pointing to the top of the shoot. This fixation of the tip meristem has the effect that the leaf spines, produced in an ascending sequence, are pulled away from the axil by continued growth in a downward and outward direction. This process also enforces the vaulting of the tubercle (podarium). He compares the whole areole to a short lateral shoot with a basal leaf rosette, stalk and terminal flower. In its juvenile stage i.e., the vegetative phase, the leaf rosette is formed as a cluster with a species-typical number of spines. Thereafter, the activity of the meristem is exhausted. In floriferous areoles the formation of a "flower-stalk zone" follows with no or with very small hair-like spines, which superficially resemble a groove. At the upper end of this "stalk", which is near the axillary end of the tubercle groove, the flower is finally produced.

Was this article helpful?

0 0

Post a comment