Nephrolepis exaltata var. bostoniensis

A beautiful American invader

The Nephrolepis exaltata is one of the garden ferns more cultivated in all the Earth regions with subtropical, Mediterranean and temperate climates without frosts. Their natural habitat is the humid forests and the marshes. It has a great facility to the vegetative reproduction through the emission of exploratory roots, which confers it an invading character that gets to turn it into a true plague when the climate and the habitat are favorable. Like the Nephrolepis cordifolia belongs to the Oleandraceae family. 

Nephrolepis exaltata of the bostoniensis variety, called Wild Boston fern, Tuber ladder fern or Fishbone fern, cultivated in the terrace of a hotel of Funchal city in the Island of Madeira. Their long, wide and turgid fronds showy differentiate from the Nephrolepis cordifolia, whose fronds are narrower and smaller. Extending the photo with a double click can be seen like rhizome has emitted long exploratory roots, of which new ferns bring forth, identical clones of its mother.

Rooting bud of the previous Nephrolepis exaltata var. bostoniensis at the beginning of May. The exploratory roots are seen that they grow looking for new lands and they are emitting rooting buds. 

The same previous roots emitting new buds to more of a meter of distance of its mother.

New frond of one of the rooting buds of the previous image. As it happens with the new fronds of the majority of the ferns, the one of the image it follows the Mathematical Sequence of Fibonacci while it is unfolded. It calls attention its dense off-white hairiness.

The Nephrolepis exaltata is an original American fern of Florida, Mexico, Cuba, Dominican Republic, Haiti, Jamaica, Puerto Rico, Brazil and French Guyana, although it has been feral and naturalized in other American countries with a warm climate without frosts, as well as in other many countries of Africa, Asia and Polynesia. In the Islands of the Macaronesia, especially in the Canary Islands, also it has been feral but it has still not become a worrisome plague. Through culture many varieties have been selected: Bostoniensis, Aurea, Chidsii, Elegantissima, Hillii, Mini Ruffle, Silver Balls, Green Fantasy, Montana, Teddy Junior, Todeoides, Whitmanii Improved, Rooseveltii, etc... The bostoniensis variety is the most commonly cultivated cultivar. This beautiful mutation was discovered in a shipment of Nephrolepis exaltata to Boston from Philadelphia in 1894.

Since I have already indicated at the beginning of this article, all the Nephrolepis has a strong invading tendency, as much through spores as through rooting buds. In the image the trunk of a canary palm is seen, Phoenix canariensis, place setting of rooting buds of Nephrolepis exaltata, arisen all from a unique unit born from a feral spore coming from a near garden, growing like epiphyte between the dried remains of the leaves of the palm. All the rooting buds are united by their roots, being in fact a single individual. The image was taken in a public garden from Orotava city in Tenerife Island.

New frond of Nephrolepis exaltata var. bostoniensis. It calls the attention the great width of its lamina. This detail difference it of the common variety of Nephrolepis exaltata, as it can be seen in the image of the previous epiphyte fern, of narrower and less vigorous fronds something. In the Nephrolepis cordifolia the fronds are even more narrower and shorter.

Frond already completely developed of Nephrolepis exaltata var. bostoniensis. It calls the attention the rachis of a shining brown mahogany color and the long pinnae with crenate edge. The fronds of this cultivar can measure between 50 and 250 centimeters in length and up to 16 centimeters in width.

Giving the return to the previous frond the gorgeous mature cleared sori filled with spores are seen, emplaced in two rows following the edge of each pinna. Pinnae have a short petiole and they are emplaced in alternating form on rachis. In its base they have two cleared auricles that do not get to embrace to rachis. This last detail difference it clearly of the Nephrolepis cordifolia, whose pinnae lack petiole and have two auricles that they embrace to rachis.

 Detail of the basal auricles of pinnae of Nephrolepis cordifolia, that they embrace to rachis and they lack petiole. This so showy difference serves to identify correctly the two more cultivated Nephrolepis in the World. 

Mature sori just before initiating the dispersion of spores. Each sorus is covered by a reniform or cleared-reniform indusium attached to the pinna on its central part as an umbrella that opens towards out allowing that the sporangia unfold and disperse spores. This kidney-shaped indusium gives the name to the sort: the Greek word Nephro means kidney and the Lepis word grudge, that is to say, grudge in the form of kidney, by the reniform form of indusium that covers the sporangia. 

Close-up image of the sori of Nephrolepis exaltata var. bostoniensis with the detail of reniform or cleared-reniform indusium as a umbrella that covers each sorus and it opens towards out. Extending the image with a double click the brown sporangia are seen very well that unfold explosively and disperse spores more far possible of their mother. On pinna are tiny brown small points that they are spores just dispersed.

Gorgeous sporangium of Nephrolepis exaltata var. bostoniensis with short sporangiophore in its inferior part that is stuck to the inferior surface of pinna and receives from it the water and the nutrients to feed spores in formation, which grow within the transparent bag embraced by the ring of cells that have a similar function to the placenta from the uterus of mammals.

Spores of Nephrolepis exaltata var. bostoniensis. Its small size inferior to 39 microns indicates that it is a diploid fern.