Read on Twitter
I will take the opportunity of these beautiful drawings from @JulianPalaeoART to talk about how cycads and bennettites branch (a thread) (1/n) https://twitter.com/JulianPalaeoART/status/1338716798682886144
Extant seed plants (and many fossils) branch from axillary buds. These are apical meristems found at the axil of new leaves, which usually lay dormant before starting to grow in response to different triggers (2/n)
Cycads do not have axillary buds. Their branching can happen either via adventitious buds (i.e. meristems that emerge from already differentiated tissue) or via dichotomous branching of the apical meristem (3/n)
In the first case, branches appear at any point on the parent trunk, but they are usually concentrated at the base. In the second case, trunks appear to bifurcate (usually multiple times) (4/n)
Recently, two articles have been published on branching in cycads, fortunately both free to access: https://doi.org/10.1086/708812
https://doi.org/10.3390/horticulturae6040065
(5/n)
https://doi.org/10.3390/horticulturae6040065
(5/n)
On the other hand, Bennettitales had axillary branching. However, their architecture seemed to be limited to a few general forms. (6/n)
In the Wielandiella-type bennettites, the main apical meristem terminated in a reproductive structure. Two branches then emerged from axillary buds continuing the growth (a phenomenon called sympodial growth). (7/n)
https://www.researchgate.net/profile/Christian-Pott/publication/283330044/figure/fig5/AS:362272452038657@1463383892111/Restoration-of-Wielandiella-angustifolia-a-plant-very-closely-related-to-Wielandiella_W640.jpg
https://www.researchgate.net/profile/Christian-Pott/publication/283330044/figure/fig5/AS:362272452038657@1463383892111/Restoration-of-Wielandiella-angustifolia-a-plant-very-closely-related-to-Wielandiella_W640.jpg
Many extant plants grow in a similar fashion, and this is recognized as an ‘architectural model’ called the Leeuwenberg model (8/n)
Williamsoniella-type and Cycadeoidea-type bennettites had different architecture: reproductive units were produced from axillary buds, while the apical meristem continued to grow (monopodial growth) (9/9)
https://www.researchgate.net/profile/Christian-Pott/publication/271914912/figure/fig3/AS:362272229740547@1463383838793/Restoration-of-Williamsoniella-coronata-from-the-Bajocian-of-Gristhorpe-Yorkshire-UK.png
https://www.researchgate.net/profile/Christian-Pott/publication/271914912/figure/fig3/AS:362272229740547@1463383838793/Restoration-of-Williamsoniella-coronata-from-the-Bajocian-of-Gristhorpe-Yorkshire-UK.png
For other details on general aspect of cycads, I have written a blogpost here: https://mariocoiro.wordpress.com/2020/10/29/palaeobotany-for-paleoartists-fossil-cycads/
TL;DR:
Cycads branch like this; Bennies branch like this.
Cycads branch like this; Bennies branch like this.
