Editing Eastern Prickly Pear (section)

== Growth Habits ==

There are basically two distinguishable growth habits observed in Connecticut's ''O. humifusa'': "individual plants" and "clonal clusters".

Individual ''O. humifusa'' plants are those which exhibit a distinct basal center.  While individual plants are almost always smaller than clonal clusters, they tend to grow taller.

Clonal clusters are large patches of ''O. humifusa'' which lack a distinct basal center and which are all linked into the same root system.  These clusters primarily result from the process of layering, whereby cladodes of a parent ''O. humifusa'' plant which contact the ground are able to grow roots of their own, producing a genetically-identical extension of the parent plant.  In this way, an individual plant may eventually expand to cover an area between 5 and 30 square feet (or more in some instances).

These two growth habits are, for the most part, readily distinguishable in their most representative instances.  However, in some cases, it can be difficult to distinguish whether or not a particular grouping of ''O. humifusa'' represents a clonal cluster or simply a series of individual plants growing close to each other.

=== Highly Variable Morphology ===

''Opuntia humifusa'' exhibits a particularly variable phenotype and, for this reason, it has been given several different scientific names across its rather broad range in the Eastern United States and Canada.  Over the years, several botanists found specimens of ''O. humifusa'' which, in one way or another, differed in appearance so drastically from those with which they were familiar that it seemed quite obvious that the new find must represent a different species.  Today, most of these erroneous species have been rejected and are understood to be synonymous with ''O. humifusa''.

Characteristics that can potentially vary from colony to colony, or even between nearby plants in the same colony, include the height of the plants, the prevailing growing habit, the shape of the cladodes, the thickness of the cladodes, the presence of spines or glochids or the lack thereof, the color of spines and glochids and the color the flowers.

Because almost every outward trait of the cactus can vary so significantly in appearance, it is possible to find two plants living in close proximity which appear as if they are indeed entirely different species.  Typically, though, plants from two different colonies on two different habitat types (especially a cactus from a coastal colony as compared to a cactus from an inland colony) will exhibit the greatest divergence in appearance from each other.  It has been suggested that, due to the fragmented distribution of ''O. humifusa'' on isolated swaths of habitat, each disjunct colony may possess unique genetic traits that differ from those found elsewhere.

=== Response to Insufficient Sunlight Exposure ===

While ''O. humifusa'' is known to be especially shade-intolerant, colonies can survive under conditions where daily exposure to direct sunlight is somewhat reduced.  In these situations, the cladodes (pads) of ''O. humifusa'' will tend to grow longer and narrower as a result of a process known as etiolation.  Etiolation occurs whenever an ''O. humifusa'' plant receives fewer hours of direct exposure to sunlight than preferred.  The elongation of the cactus pads is an attempt of the cactus to "reach" towards sunlight, similar to the way in which flower species germinating in the shade will grow long, arching stems in an attempt to reach the sunlight.<ref name="cambridge">Nobel, Park S. Environmental Biology of Agaves and Cacti. Cambridge: Cambridge University Press, 2003. 161. Print. <http://books.google.com/books?id=jdO2U84I6JwC&pg=PA161&dq=humifusa+cactus&hl=en&sa=X&ei=-ePET5mWJerDsQKy9ZzQCQ&ved=0CDcQ6AEwADgU#v=onepage&q=humifusa%20cactus&f=false>.</ref>

[[File:Etiolated-opuntia-humifusa-longbeachcomplex.JPG|thumb|left|Tightly surrounded by ''Rosa rugosa'' (Beach Rose), the ''Opuntia humifusa'' seen here at the Long Beach Complex in Stratford, CT has developed a growth habit indicative of etiolation, whereby it has grown especially long cladodes in an effort to gain improved sunlight exposure.]]Wherever etiolation is observed in ''O. humifusa'', the plant is not receiving an ideal amount of direct sunlight exposure.  The [[#Ingham Hill Colony (Old Saybrook)|Ingham Hill Colony]] and [[#West Rock North Complex|West Rock North Complex]], for example, exhibit particularly long cladodes (shaped like an elongated ellipse) that are undoubtedly undergoing etiolation in an attempt reach toward sunny areas that aren't shaded out by larger trees.  In contrast, the ''O. humifusa'' of the Long Beach Complex and Short Beach Colony generally have rather short cladodes which are occasionally almost circular in their profile, owing to the fact that they receive plenty of direct sunlight exposure and are not undergoing etiolation, at all.

Although etiolation is a symptom of insufficient habitat conditions, it is not known how long ''O. humifusa'' can persist under etiolation conditions.  It is possible, though not verified, that a colony may persist for decades under reduced light conditions and possess a nearly permanent etiolated growth habit.  However, under reduced-light conditions, ''O. humifusa'' has been observed to cease flowering<ref name="ohio-study"></ref>, a scenario in which the colony will not be able expand rapidly or disperse beyond its clonal cluster.

==== Gauging Severity of Etiolation ====

[[File:Opuntia-unusual-cladodes-1.jpg|thumb|right|Despite receiving several hours of direct sunlight each day, this cactus of the Long Beach Complex in Stratford, CT exhibits peculiarly elongated cladodes.  This unusual morphology is ''not'' caused by etoliation, but must represent some other genetic or environmental influence.]]Etiolation in ''O. humifusa'' results in cladodes that are much longer than they are wide.  However, this length-to-width ratio cannot exclusively be used to judge the degree of etiolation of a given specimen.  In certain cases, I have discovered specimens growing on warm, exposed sand dunes (receiving no less than 6-8 hours of unobstructed sunlight per day) which exhibit unusually long, skinny cladodes that cannot have occurred due to etiolation, but which must have resulted from some other combination of genetic and/or environmental factors.

Therefore, in determining if a given ''O. humifusa'' specimen with elongated cladodes is suffering from insufficient sunlight exposure, it is necessary to consider the conditions of its immediate habitat.  Examples of instances in which I have discovered etiolated ''O. humifusa'' specimens are as follows:

<ul>
<li>'''Cacti growing within a thick patch of coastal ''Rosa rugosa'' (Beach Rose)'''.  In this case, the cacti probably colonized the area before the arrival of ''R. rugosa'', which would have quickly overtaken the cacti.  In response, the "buried" cacti were growing especially long, rigid cladodes in an effort to hoist themselves towards sunlight.  These cacti, which still seemed surprisingly healthy, had already exceeded what is typically thought of as the maximum height of this usually prostrate species.</li>
<li>'''Cacti growing upon a tight, rocky glade within the forest.'''  In the case of the Ingham Hill Colony in Old Saybrook, the ''O. humifusa'' are being gradually shaded out by encroaching canopy growth.  These specimens all exhibit rather elongated cladodes, certainly indicative of etiolation due to the miniminal amount of sunlight exposure available.</li>
</ul>

In general, elongated cladode length on any given specimen should only be attributed to etiolation in those instances where the affected plant clearly cannot obtain 6 hours of relatively unobstructed sunlight per day.